Musculoskeletal Amboss Q&A

Ligaments of the knee

A "A" refers to the anterior cruciate ligament (ACL), the most commonly injured knee ligament. The mechanism of this patient's injury is typical for damage to the ACL: Sudden deceleration and twisting to change direction as occurs during pivoting movements (e.g., "cutting inwards"). This results in an audible pop followed by knee swelling (i.e., hemarthrosis) and joint instability. Clinical tests for injury to the ACL (e.g., the Lachman test, anterior drawer test, and pivot shift test) assess for laxity of the injured ligament compared to the contralateral side.

An investigator is studying the microstructure of skeletal muscle fibers to identify new targets for skeletal muscle relaxation. He obtains a biopsy from the brachioradialis of a test subject and observes it under a transmission electron microscope. A photomicrograph of the structures seen during microscopy is shown. Which of the following parts contains the greatest proportion of actin filaments? The structure in question decreases in length during muscle contraction.

A This structure is the I band, a region of the sarcomere that contains the greatest proportion of actin filaments. During contraction, the power stroke mechanism of myosin heads form a crossbridge with actin that pulls the thin filament along the myosin (the thick filament) toward the M line, decreasing the width of the I band.

An investigator is studying muscle contraction in tissue obtained from the thigh muscle of an experimental animal. After injection of radiolabeled ATP, the tissue is stimulated with electrical impulses. Radioassay of these muscle cells is most likely to show greatest activity in which of the following structures? Activity will be greatest at the site of cross-bridge formation.

A band The outer segments of the A band are the sites where actin and myosin filaments overlap and form cross-bridges via the myosin heads, which bind and hydrolyze ATP. Following injection of radiolabeled ATP, radioassay is most likely to show the greatest activity in the A band of the sarcomere.

An investigator is studying the crossbridge cycle of muscle contraction. Tissue from the biceps brachii muscle is obtained at the autopsy of an 87-year-old man. Investigation of the muscle tissue shows myosin heads attached to actin filaments. Binding of myosin heads to which of the following elements would most likely cause detachment of myosin from actin filaments? Absence of which of the following is the underlying cause of rigor mortis?

ATP Binding of ATP to myosin heads causes myosin to detach from actin filaments, which disrupts the crossbridge cycle and ends muscle contraction. Hydrolysis of myosin-bound ATP to ADP and Pi, on the other hand, alters the conformation of myosin heads to a cocked state, which primes the myosin for crossbridge formation with actin to generate muscle contraction.

A 41-year-old woman comes to the doctor because of gradually progressive weakness in her right hand over the past few weeks. She goes to the gym to lift weights 5 times a week. With the dorsum of the right hand on a flat surface, the patient is unable to move her thumb to touch a pen held 2 cm above the interphalangeal joint of the thumb. An MRI of the right arm shows compression of a nerve that passes through the pronator teres muscle. Based on the examination findings, loss of innervation of which of the following muscles is most likely in this patient? This patient presents with an inability to abduct the thumb (pen test). This finding indicates weakness of a muscle that is innervated by the median nerve, which runs through the two heads of the pronator teres muscle.

Abductor pollicis brevis This patient shows signs of weakness of the abductor pollicis brevis. The abductor pollicis brevis is a muscle of the thenar eminence that arises from the flexor retinaculum and the scaphoid bone, before inserting into the proximal phalanx and the capsule of the first metacarpophalangeal joint. This muscle is responsible for thumb abduction, extension, and opposition. Damage to the median nerve leads to a weakness of these functions, especially abduction, which can be assessed by the pen test. Repetitive activities such as weight-lifting can cause median nerve injury by direct compression or entrapment.

A 3855-g (8-lb 8-oz) newborn is examined shortly after birth. She was delivered at 40 weeks' gestation by cesarean delivery because of breech presentation. Pregnancy was otherwise uncomplicated. Physical examination shows asymmetric thigh creases. The left leg is shorter than the right leg and positioned in external rotation. Which of the following is the most likely underlying cause of this patient's findings? Further examination of this patient will likely reveal a palpable clunk when the left hip is abducted, flexed, and lifted forward (Ortolani sign) as well as a similar clunk when posterior pressure is applied on the flexed hip (Barlow sign). These are signs of hip instability.

Abnormal development of the acetabulum Abnormal development of the acetabulum and an abnormal relationship between this structure and the femoral head is the underlying cause of developmental dysplasia of the hip (DDH). Depending on the affected structures, DDH can cause hip instability (capsular laxity), subluxation of the femoral head, hip dislocation, and/or acetabular dysplasia. The etiology is not yet fully understood, but a breech presentation, female sex, firstborn status, and a family history of DDH are important risk factors. Early detection of DDH is important, as the prognosis worsens with increasing age. Hip ultrasound is the imaging modality of choice in all infants younger than 4 months. A harness is the treatment of choice for infants with DDH who are less than 6 months old.

A 53-year-old multiparous woman is scheduled to undergo elective sling surgery for treatment of stress incontinence. She has frequent loss of small amounts of urine when she coughs or laughs, despite attempts at conservative treatment. The physician inserts trocars in the obturator foramen bilaterally to make the incision and passes a mesh around the pubic bones and underneath the urethra to form a sling. During the procedure, the physician accidentally injures a nerve in the obturator foramen. The function of which of the following muscles is most likely to be affected following the procedure? The nerve passing through the obturator foramen is the obturator nerve, which is at risk of being injured during pelvic surgeries or as a result of pelvic trauma.

Adductor longus The obturator nerve innervates the muscles of the medial compartment of the thigh, including the adductor longus, adductor magnus, adductor brevis, gracilis, external obturator, and, occasionally, the pectineus muscle. It is frequently damaged during midurethral sling surgeries, as well as by pelvic ring fractures and obturator hernias. Damage to this nerve can cause sensory deficits along the medial thigh and weakness of hip adduction.

A 68-year-old woman is brought to the emergency room because of pain in her right shoulder after falling down the stairs in her apartment. She has a history of postmenopausal osteoporosis. Her current medications are alendronate, vitamin D, and calcium supplementation. Physical examination shows shortening of the right arm as well as tenderness and swelling over the right shoulder. The range of motion of the right arm is limited by pain. An x-ray of the right shoulder shows a fracture of the surgical neck of the humerus. Which of the following movements is most likely to be impaired in this patient? Examination would likely also show flattening of the deltoid prominence.

Arm abduction Impaired arm abduction over 15 degrees due to weakness of the deltoid muscle is a classic finding of axillary nerve injury. The axillary nerve originates from the posterior cord of the brachial plexus and carries fibers from C5-6 through the quadrangular space to innervate the deltoid. Due to its proximity to the surgical neck of the humerus, the nerve can become injured by a proximal humerus fracture, as seen in this patient. Further symptoms of axillary nerve injury include impaired external rotation of the arm (due to paralysis of the teres minor muscle) and sensory loss over the lateral shoulder.

A 54-year-old woman comes to the physician because of a 3-month history of upper midthoracic back pain. The pain is severe, dull in quality, and worse during the night. Ten months ago, she underwent a modified radical mastectomy for invasive ductal carcinoma of the right breast. Physical examination shows normal muscle strength. Deep tendon reflexes are 2+ in all extremities. Examination of the back shows tenderness over the thoracic spinous processes. An x-ray of the thoracic spine shows vertebral osteolytic lesions at the levels of T4 and T5. The patient's thoracic lesions are most likely a result of metastatic spread via which of the following structures? This patient has metastatic lesions in the vertebrae due to spread of her breast cancer to a structure that communicates with the paravertebral plexus of Batson.

Azygos vein The azygos vein receives blood from the right intercostal veins, which in turn receive blood from the right breast. The azygos vein has numerous anastomoses with the vertebral venous plexus, which is a longitudinal network of valveless veins in the spine that includes the paravertebral plexus of Batson. The lack of valves in this network facilitates the spread of tumors and infections. Spinal metastases from breast cancer most commonly develop in the thoracic vertebrae. Metastases to bone are most commonly the result of hematogenous spread.

A 31-year-old man comes to the physician because of a 1-month history of pain in his right arm and numbness and tingling in his right hand. One month ago, he quit his job as a painter because the symptoms worsened whenever he tried to reach above his head. He has no history of serious illness and takes no medications. He does not smoke and drinks two beers daily. Vital signs are within normal limits. Physical examination shows decreased sensation to fine touch in the 4th and 5th fingers of his right hand. Radial pulse intensity decreases when the patient's right arm is abducted and externally rotated. Which of the following is the most likely cause of this patient's symptoms? Physical examination shows a positive Adson test (i.e., decreased pulse intensity with arm abduction and external rotation).

Cervical rib A cervical rib is a common cause of thoracic outlet syndrome, which occurs because of compression of the lower trunk of the brachial plexus (derived from the C8-T1 nerve roots) and/or subclavian vessels in the area of the scalene triangle. Compression of the lower trunk of the brachial plexus can cause neuropathic pain, paresthesias, and sensory loss in the C8 and T1 dermatomes (i.e., 4th and 5th fingers, medial aspect of the arm), as seen here, and possibly weakness of the intrinsic muscles of the hand. The symptoms in the case of a cervical rib are usually exacerbated by overhead arm elevation. Compression of the subclavian vessels is less common; subclavian vein compression results in swelling of the arm, and subclavian artery compression can cause acute or chronic limb ischemia and, possibly, subclavian steal syndrome. A positive Adson test, as seen here, is suggestive of a cervical rib. Other common causes of thoracic outlet syndrome are Pancoast tumor, muscular anomalies (e.g., supernumerary scalene muscles, neck muscle hypertrophy), and trauma (e.g., clavicle fracture).

A 13-year-old boy is brought to the emergency room 30 minutes after being hit in the face with a baseball at high velocity. Examination shows left periorbital swelling, posterior displacement of the left globe, and tenderness to palpation over the left infraorbital rim. There is limited left upward gaze and normal horizontal eye movement. Further evaluation is most likely to show which of the following as a result of this patient's trauma? The patient's findings of enophthalmos (posteriorly displaced eye) and infraorbital rim tenderness are suggestive of an orbital floor fracture (orbital blowout fracture). The mechanism of this injury can lead to downward displacement of orbital fat and inferior rectus muscle entrapment, which in turn can cause impaired upward gaze.

Clouding of maxillary sinus Clouding of the maxillary sinus is a radiological sign known as the "tear drop sign." It will most likely be seen on further evaluation due to the inferior rectus muscle and orbital fat herniating through the fractured orbital floor. Other symptoms may include diplopia, crepitus, ecchymosis, and loss of sensation in the area innervated by the infraorbital nerve (midface from the lower eyelid to the upper lip).

A 40-year-old woman is brought to the emergency department after falling down the stairs in her house. She began taking levofloxacin 4 days ago for community-acquired pneumonia and takes no other medications. Physical examination shows severe tenderness, swelling, erythema, and decreased range of motion of her left ankle joint. An MRI of her left ankle shows a partial tear in the anterior talofibular ligament and a complete tear in the tendon of the muscle that originates from the lower two-thirds of the fibula's lateral surface and inserts onto the base of the fifth metatarsal bone. The affected muscle normally aids in which of the following foot movements? This movement would be impaired if the superficial peroneal nerve was injured.

Eversion The peroneus brevis muscle, located in the lateral compartment of the leg, is primarily responsible for eversion of the foot and assists in plantarflexion of the foot. Injury manifests primarily with impaired foot eversion while plantarflexion may be normal or weak. Rupture of the tendon is an uncommon severe complication of fluoroquinolones (e.g., levofloxacin), particularly in older patients.

Seven hours after undergoing left hip arthroplasty for chronic hip pain, a 67-year-old woman reports a prickling sensation in her left anteromedial thigh and lower leg. Neurologic examination shows left leg strength 3/5 on hip flexion and 2/5 on knee extension. Patellar reflex is decreased on the left. Sensation to pinprick and light touch are decreased on the anteromedial left thigh as well as medial lower leg. Which of the following is the most likely underlying cause of this patient's symptoms? Postoperative anteromedial thigh and lower leg paresthesias with concurrent decreased patellar reflex, hip flexion, and knee extension are consistent with damage to a peripheral nerve of the anterior lower extremity.

Femoral nerve injury This patient has evidence of femoral nerve injury, likely from prolonged compression during surgery. Injury of the femoral nerve typically results in loss of sensation in the anteromedial thigh and medial lower leg (anterior cutaneous branches), as well as decreased strength in hip flexion (iliopsoas muscle) and knee extension (quadriceps femoris muscle).

A 37-year-old man is brought to the emergency department after being attacked with a knife. Physical examination shows a 4-cm laceration in the midline of the right forearm. An MRI of the right arm shows damage to a nerve that runs between the superficial and deep flexor digitorum muscles. Loss of sensation over which of the following areas is most likely in this patient? The median nerve, which originates from the lateral and medial cords of the brachial plexus, passes down the arm into the cubital fossa and runs in the midline of the ventral compartment of the forearm between the flexor digitorum superficialis and flexor digitorum profundus.

Fingertip of the index finger The median nerve passes through the carpal tunnel into the hand. Within the hand, the median nerve motor branch innervates the thenar muscles and the lateral two lumbricals. The median nerve also provides sensory innervation to the thenar eminence as well as the dorsal and palmar aspect of the lateral 3½ fingers including the fingertips. Note that the lateral border of the thenar eminence is innervated by the radial nerve.

A 28-year-old man comes to the physician because of a 1-week history of weakness in the fingers of his right hand. One week ago, he experienced sudden pain in his right forearm during weight training. He has no history of serious illness. Physical examination shows impaired flexion of the proximal interphalangeal joints, while flexion of the distal interphalangeal joints is intact. Which of the following muscles is most likely injured? The affected muscle splits into four tendons to attach to the middle phalanges of the second to the fifth digit.

Flexor digitorum superficialis The flexor digitorum superficialis muscle, which originates from the medial epicondyle of the humerus and proximal shaft of the radius and inserts into the middle phalanges of the fingers, enables flexion of the wrist, metacarpophalangeal joints, and proximal interphalangeal joints. Isometric weight training can cause rupture of this muscle, as seen here. Flexion of the distal interphalangeal joints is a function of the flexor digitorum profundus muscle and is unaffected by flexor digitorum superficialis muscle injury.

A 19-year-old man comes to the physician because of episodic tingling and numbness in his right hand for the past 3 weeks. He says that he cannot hold a pen very well and that this is affecting his handwriting. The symptoms started after he played in a football game. Physical examination shows decreased pinch strength in the right hand. Sensation to light touch is decreased over the right little finger and the medial palmar surface of the right hand. Radiologic evaluation in this patient is most likely to show a fracture in which of the following bones? Decreased pinch strength and sensory deficits over the little finger and palmar surfaces of the medial aspect of the right hand are suggestive of ulnar nerve entrapment at the level of the wrist.

G Blunt trauma to the hook of hamate (e.g., fall on an outstretched hand) can lead to entrapment of the ulnar nerve, which is referred to as a distal ulnar lesion. In addition to paresthesias, numbness, weakness, and pain, this lesion can also cause claw hand deformity.

A 49-year-old man comes to the physician because of severe, shooting pain in his lower back for the past 2 weeks. The pain radiates down the back of both legs and started after he lifted a concrete manhole cover from the ground. Physical examination shows decreased sensation to light touch bilaterally over the lateral thigh area and lateral calf. Patellar reflex is decreased on both sides. The passive raising of either the right or left leg beyond 30 degrees triggers a shooting pain down the leg past the knee. Which of the following is the most likely underlying cause of this patient's current condition? Severe lower back pain radiating down the legs together with decreased sensation to touch, decreased reflexes, and positive Lasègue sign (pain when raising legs passively) is suggestive of lumbar radiculopathy. The association between heavy lifting and the onset of symptoms suggests an acute, mechanical mechanism of injury.

Herniation of nucleus pulposus into the vertebral canal The nucleus pulposus is the gelatinous core of the intervertebral disc and is surrounded by a dense outer ring, the annulus fibrosus. Compression, tension, and shear and torque stresses on the spinal disc (as with heavy lifting) together with degenerative changes (e.g., dehydration) can lead to a tear in the annulus fibrosus, resulting in herniation of the nucleus pulposus into the vertebral canal with subsequent compression of the spinal cord and/or nerve roots. This nerve impingement causes symptoms of radiculopathy, which differ depending on the level of herniation.

A 45-year-old man is brought to the emergency department following a motor vehicle collision. He reports right hip pain and numbness along the right thigh. Physical examination shows decreased sensation to light touch over a small area of the medial thigh. X-rays of the pelvis show a displaced pelvic ring fracture. Further evaluation of this patient is most likely to show which of the following findings? This man has a pelvic ring fracture with decreased sensation over a small area of the right medial thigh. This is consistent with an obturator nerve injury.

Impaired adduction of the hip Pelvic ring fracture is a common cause of obturator nerve injury. In addition to providing sensory cutaneous innervation to a small area of the medial thigh, the obturator nerve also provides motor innervation to the hip adductor muscles in the medial compartment of the thigh. Thus, injury to the nerve can result in impaired adduction of the hip.

A 28-year-old man comes to the physician because of a 3-month history of pain in his left shoulder. He is physically active and plays baseball twice a week. The pain is reproduced when the shoulder is externally rotated against resistance. Injury of which of the following tendons is most likely in this patient? The rotator cuff tendons, which help to stabilize the glenohumeral joint through its broad range of motion, are commonly injured in overhead throwing athletes.

Infraspinatus Overhead throwing activities (e.g., baseball pitching) place supraphysiological loads on the tendons of the rotator cuff. Through eccentric contraction, this type of activity especially strains the posterior rotator cuff when slowing down after releasing the ball. Tendon overload can cause microscopic tears called microinjuries. The relatively avascular nature of tendons means that these microinjuries have limited healing potential and can propagate into partial or full-thickness tears, or tendinopathy. The infraspinatus muscle, along with the teres minor muscle, is responsible for the external rotation of the humerus. This patient's shoulder pain during external rotation suggests infraspinatus tendinopathy.

A 25-year-old woman comes to the physician because of pain and weakness in her left leg that started 2 days ago while running a marathon. Physical examination shows impaired flexion of the left knee joint and impaired extension of the left hip joint. An MRI of the left lower extremity shows injury to the long head of the biceps femoris muscle and the semitendinosus muscle. Which of the following is the most likely origin of the injured muscles? The inferior gemellus, adductor magnus, and semimembranosus muscles also attach to this anatomical landmark.

Ischial tuberosity 60% The long head of the biceps femoris and the semitendinosus muscle originate from the ischial tuberosity. The muscle belly of the long head of the biceps femoris joins together with the short head of the biceps femoris to form a conjoined tendon, which inserts primarily into the lateral aspect of the fibular head.ness in her left leg that started 2 days ago while running a marathon.

A 36-year-old woman comes to the physician because of a 2-month history of progressively worsening lower back pain and weakness in the lower extremities. The pain is worse with movement and improves with lying down on a flat surface. She was diagnosed with pulmonary tuberculosis 6 months ago and is currently taking isoniazid and rifampin. Physical examination shows sensory loss over the anterolateral aspect of the lower-thigh, patella, and medial aspect of the right lower leg. Strength is 2/5 with knee extension and the patellar reflex is absent. An x-ray of the spine shows a paravertebral opacity with anterior wedging of the vertebral body. Which of the following nerve roots is most likely to be affected in this patient? A history of radiculopathy symptoms in a patient with primary tuberculosis and x-ray findings of anterior wedging of the vertebral bodies and paravertebral opacity is suggestive of spinal tuberculosis (Pott disease).

L4 L4 radiculopathy characteristically manifests with sensory loss or paresthesia extending from the distal anterolateral thigh over the patella to the medial lower leg, as seen in this patient. Diminished knee extension would also be expected since innervation of the quadriceps is impaired. This patient's reduced patellar reflex further supports the diagnosis of impingement of the L4 nerve root as the underlying cause.

A 19-year-old man is brought to the emergency department because of severe right shoulder pain and inability to move the shoulder after a fall while climbing outdoors. Examination shows the right upper extremity is externally rotated and slightly abducted. There is loss of the the normal rounded appearance of the shoulder. The right humeral head is palpated below the coracoid process. Sensation to pinprick over which of the following skin areas is most likely to be reduced? A combination of external rotation and abduction of the arm, flattening of the shoulder contour, and palpation of the humeral head below the coracoid process is indicative of anterior shoulder dislocation. The axillary nerve is stretched over the displaced head of the humerus (neurapraxia), making it especially prone to injury.

Lateral aspect of the shoulder The nerve most commonly damaged as a result of anterior shoulder dislocation is the axillary nerve, which has a dermatome over the lateral shoulder. This injury thus often causes loss of sensation in this area.

A 54-year-old woman comes to the emergency department because of drooping on the left side of her face since that morning. She also reports difficulty closing her eyes and chewing. During the neurologic examination, the physician asks the patient to open her jaw against resistance. Which of the following muscles is most likely activated in this movement? The muscle responsible for lowering the jaw has two heads and is innervated by the mandibular division of the trigeminal nerve.

Lateral pterygoid The temporalis, medial pterygoid, lateral pterygoid, and masseter are the four primary muscles of mastication, which are responsible for the movement of the mandible during the process of chewing. The lateral pterygoid is the only muscle that lowers the mandible (opens the jaw), while the other three muscles function to close the jaw. Together with the medial pterygoid muscle, the lateral pterygoid also functions to move the mandible from side to side (contraction of pterygoids causes deviation to contralateral side). All of the muscles of mastication are innervated by the mandibular division of the trigeminal nerve (V3).

An 80-year-old woman is brought to the emergency department for left hip pain 30 minutes after she fell while walking around in her room. Examination shows left groin tenderness. The range of motion of the left hip is limited because of pain. An x-ray of the hip shows a linear fracture of the left femoral neck with slight posterior displacement of the femur. Which of the following arteries was most likely damaged in the patient's fall? The artery that courses posterior to the femoral neck is at greatest risk of injury in this case. Damage to this artery can cause avascular necrosis of the femoral head.

Medial circumflex femoral The medial circumflex femoral artery is a major branch of the profunda femoris. It courses medioposteriorly to the neck of the femur, where it supplies blood to the head and neck of the femur. A minority of femoral neck fractures will go on to develop osteonecrosis of the femoral head stemming from injuries to this artery and its small branches to the bone.

A 21-year-old man comes to the physician for a follow-up examination. Four days ago, he injured his right knee while playing soccer. Increased laxity of the right knee joint is noted when the knee is flexed to 30° and an abducting force is applied to the lower leg. The examination finding in this patient is most likely caused by damage to which of the following structures? The patient's injury is likely to be one of the injuries that comprise the unhappy triad.

Medial collateral ligament Injury to the medial collateral ligament would cause increased laxity of the knee joint when the knee is flexed and an abducting force is applied to the lower leg (valgus stress test). Medial collateral ligament injury is often associated with injury to the anterior cruciate ligament and medial meniscus (unhappy triad).

26-year-old woman comes to the physician because of severe pain in her right wrist one day after falling onto her hands and knees while rollerskating. Physical examination shows abrasions over the knees and bruising over the volar aspect of the right wrist. There is swelling and tenderness on palpation of the volar wrist joint, as well as restricted range of motion due to pain. An x-ray of the hand shows volar dislocation of the lunate bone. Further evaluation is most likely to show which of the following? Volar dislocation of the lunate bone can cause compression of structures within the carpal tunnel.

Paresthesia over the volar aspect of the first 3 fingers on wrist flexion Paresthesia over the volar aspect of D1, D2, and D3 on wrist flexion (e.g., positive Phalen test) is consistent with median nerve injury, which can occur due to carpal tunnel compression from lunate dislocation. Lunate dislocations are most commonly the result of falling onto a hyperextended and ulnar-deviated wrist and typically present with acute wrist pain and swelling and tenderness to palpation of the volar wrist, as seen in this patient. Other complications related to lunate dislocation include avascular necrosis.

An 18-year-old man is brought to the emergency department 30 minutes after being stabbed in the chest during a fight. He has no other injuries. His pulse is 120/min, blood pressure is 90/60 mm Hg, and respirations are 22/min. Examination shows a 4-cm deep, straight stab wound in the 4th intercostal space 2 cm medial to the right midclavicular line. The knife most likely passed through which of the following structures? ed by a line originating at a point 2 cm medial to the midclavicular line.

Pectoral fascia, transversus thoracis muscle, right lung The pectoralis major, sheathed in its fascia, spans the anterior chest wall, originating from the medial clavicle, anterior sternum, and six upper costal cartilages, and inserting into the proximal humerus. The transversus thoracis muscles lie on the internal surface of the thoracic cage, originating from the sternum and spanning out laterally to the 2nd to 6th ribs. The pectoralis fascia, transversus thoracis, and right lung would be intersect

A 56-year-old man is brought to the emergency department 30 minutes after falling from a height of 3 feet onto a sharp metal fence pole. He is unconscious. Physical examination shows a wound on the upper margin of the right clavicle in the parasternal line that is 3-cm-deep. Which of the following is the most likely result of this patient's injury? This condition can also be caused by improper placement of a central venous catheter.

Pneumothorax due to pleural injury The pleural membrane covering the apex of the superior lobe of the lung extends approx. 2.5 cm above the medial end of the clavicle. A penetrating injury to the supraclavicular parasternal region, like the one sustained by this patient, can pierce the parietal pleural membrane, permitting air to enter the pleural space and causing an ipsilateral pneumothorax.

A 27-year-old man comes to the physician because of pain and swelling in his right knee that began 3 days ago when he fell during football practice. He fell on his flexed right knee as he dove to complete a pass. He felt some mild knee pain but continued to practice. Over the next 2 days, the pain worsened and the knee began to swell. Today, the patient has an antalgic gait. Examination shows a swollen and tender right knee; flexion is limited by pain. The right knee is flexed and pressure is applied to proximal tibia; 8 mm of backward translation of the foreleg is observed. Which of the following is most likely injured? Flexion of the quadriceps of the right thigh would translate the tibia forward; this is called a positive quadriceps active test and it is another sign of the patient's condition.

Posterior cruciate ligament Posterior cruciate ligament (PCL) injury can occur when the flexed knee is subjected to excessive posterior force, as in the case of this patient. He has a positive posterior drawer sign on the affected side, which is highly suggestive of PCL injury. PCL injury is confirmed by MRI and can be treated conservatively (in the case of isolated injury) or surgically (in the case of multi-ligament injury or chronic symptoms).

A 25-year-old woman comes to the physician because of pain and weakness in her right forearm and hand for several months. Two years ago, she sustained a fracture of her ulnar shaft with dislocation of the radial head that was treated surgically. Physical examination shows mild tenderness a few centimeters distal to the lateral epicondyle. She has marked weakness when attempting to extend her right middle finger. There is radial deviation on extension of the wrist. Sensation is not impaired. Which of the following nerves is most likely affected in this patient? This patient's weakness in her wrist extensor muscles with intact cutaneous sensation suggests a pure motor neuropathy that is sometimes associated with dislocation of the radial head.

Posterior interosseous nerve The posterior interosseous nerve is a motor nerve that branches off from the radial nerve in close proximity to the radial head, which makes it susceptible to injury due to radial head dislocation or subluxation. Occasionally, posterior interosseous neuropathy can develop years after a fracture of the forearm or elbow. Compression of this nerve frequently impairs extension of the fingers (more than the wrist) because function of the extensor carpi radialis longus, which is innervated directly by the radial nerve, is preserved. The unopposed action of this muscle further explains the radial deviation seen on examination. Finally, this patient's normal sensory examination is one of the hallmarks of posterior interosseous neuropathy because this nerve does not provide cutaneous innervation.

A 38-year-old man comes to the physician because of a 3-week history of right-sided knee pain. He works as a bricklayer and reports that the pain worsens when he kneels. He has no history of trauma. Examination of the right knee shows erythema, fluctuant swelling, and tenderness on palpation of the kneecap. Passive flexion of the right knee elicits pain. Which of the following structures is most likely affected in this patient? This patient's occupational history puts him at risk for an overuse injury known as housemaid's knee.

Prepatellar bursa Inflammation of the prepatellar bursa, which is located between the skin and kneecap, can be caused by repeated flexion of the knee joint or recurrent trauma. Individuals with jobs that require frequent kneeling (e.g., bricklayers, plumbers, mechanics) are especially prone to developing prepatellar bursitis. The condition commonly manifests with localized erythema, fluctuant swelling (the inflamed bursa), and tenderness to palpation and movement of the kneecap, all of which are observed in this patient. Surgery may be indicated (e.g., drainage of pus, bursectomy) for recurrent bursitis if conservative management fails.

A 3-year-old boy is brought to the physician because of a 3-day history of fever and chills. The mother reports that he has also been limping for 2 days. He has no history of trauma to this region. His temperature is 38.9°C (102°F). Physical examination shows dull tenderness over his left lower extremity. The range of motion of the leg is also limited by pain. While walking, he avoids putting weight on his left leg. Laboratory studies show an erythrocyte sedimentation rate of 67 mm/h. An MRI is most likely to show abnormalities in which of the following regions? Fever, chills, limping, local tenderness, and the elevated ESR in this young boy raise concerns for osteomyelitis. Acute osteomyelitis in children is usually caused by hematogenous spread; a particular region of the bone is especially susceptible to the deposition of bacteria.

Proximal metaphysis of the femur Hematogenous osteomyelitis is most common in children and adolescents (> 50% of cases occur in children ≤ 5 years). The metaphyseal blood supply is unique, in that the nutrient artery supplying the metaphysis makes a sharp loop before it empties into sinusoidal spaces, where the rate of blood flow is decreased. This creates a medium for bacteria to proliferate. The focus of infection in the metaphysis leads to cellulitis in the bone marrow. Inflammation in the marrow causes increased intramedullary pressure, which forces the exudate into the cortex, where it can rupture through the periosteum. More than 80% of cases of hematogenous osteomyelitis occur in the long bones, the femur and tibia being the most common, and cases usually originate in the metaphyses.

A 35-year-old woman comes to the physician because of progressive left flank pain over the past 2 weeks. She has a history of type 1 diabetes mellitus. Her temperature is 38°C (100.4°F). There is tenderness to percussion along the left flank. Passive extension of the left hip is painful. Her leukocyte count is 16,000/mm3. An axial CT scan is shown. The underlying pathology is most likely located in which of the following anatomical structures? The CT scan shows a left-sided, circular, isodense structure with clear borders. In combination with fever, flank pain, and leukocytosis, these findings are suggestive of abscess formation.

Psoas major muscle This patient's subacute flank pain, positive psoas sign, fever, and leukocytosis are suggestive of an abscess that involves the hip flexors (e.g., psoas major, iliacus, or iliopsoas). The CT scan indicates an abscess within the left psoas major muscle. This muscle originates from the vertebral bodies, intervertebral discs, and transverse processes of T12-L5. A psoas abscess can be classified as either a primary abscess due to hematogenous spread of infection (e.g., bacteremia, endocarditis), or a secondary abscess caused by local spread of infection (e.g., osteomyelitis, pyelonephritis, trauma). Primary abscesses are more common than secondary abscesses.

A previously healthy 2-year-old boy is brought to the emergency room by his mother because of persistent crying and refusal to move his right arm. The episode began 30 minutes ago after the mother lifted him up by the arms. He appears distressed and is inconsolable. Vital signs are within normal limits. On examination, his right arm is held close to his body in a flexed and pronated position. Which of the following is the most likely diagnosis? This patient presents with the most common elbow injury under 5 years of age. It is caused by sudden axial traction of the pronated and extended forearm.

Radial head subluxation Radial head subluxation describes the slipping of the annular ligament over the radial head. Patients classically present with the arm held in pronated and flexed position, as in this scenario. It usually occurs when adults pull on children's arms to hold them back or to swing them around. The radial head is relocated by hyperpronation or supination of the flexed forearm.

A 21-year-old woman is brought to the emergency department following a motor vehicle collision. She has significant pain and weakness in her right arm and hand. Physical examination shows multiple ecchymoses and tenderness in the right upper extremity. She is able to make a fist, but there is marked decrease in grip strength. An x-ray of the right upper extremity shows a midshaft humerus fracture. Which of the following structures is most likely injured? This patient most likely cannot extend her hand at the wrist joint.

Radial nerve Radial nerve injury is the most common complication associated with humerus mid-shaft fractures due to the radial nerve's proximity to the humerus shaft. It is typically associated with wrist drop, decreased finger extensor strength, and loss of sensation over the dorsal hand and forearm. Grip strength can also be decreased through impaired wrist extension, as seen in this patient, but the ability to make a fist is not affected.

A 45-year-old man comes to the physician because of mild left-sided abdominal pain when lifting heavy objects that has developed over the past 6 months. He has had a weight loss of 41 kg (90 lb) in the past year. Physical examination shows a soft bulge inferior and lateral to the navel on the left. A CT scan of the abdomen of a patient with the same condition is shown. Which of the following muscles is located directly medial to the pathology seen on the CT scan? The CT scan shows a left-sided Spigelian hernia. Rapid weight loss is a risk factor for this condition

Rectus abdominis The rectus abdominis muscle lies directly medial to the hernia sac on this CT scan. Spigelian hernias occur through defects in the linea semilunaris of the anterior abdominal wall. The linea semilunaris is a curved, vertical line at the lateral edge of each rectus abdominis muscle that is composed of the aponeurotic fibers of the internal oblique muscle as they merge with the rectus sheath. Spigelian hernias are therefore bounded medially by the rectus abdominis. These hernias are occasionally palpable as a subcutaneous bulge. However, they are most commonly intraperitoneal and may not be evident on examination. Because the fascial defects are typically small, there is a high risk of bowel strangulation; surgical repair is therefore indicated.

A 36-year-old woman comes to the physician because of a 2-week history of progressively worsening pain on the outer side of her left elbow. She does not recall any trauma to the area. The patient plays badminton recreationally. Examination shows tenderness over the lateral surface of the left distal humerus. The pain is reproduced by supinating the forearm against resistance. Which of the following is the most likely underlying cause of this patient's pain? Progressively worsening pain near the lateral epicondyle in a racket-sports enthusiast suggests lateral epicondylitis (tennis elbow).

Repeated wrist extension The lateral epicondyle is the common origin of the wrist extensors. Lateral epicondylitis is caused by activities, e.g., racket sports such as tennis and badminton, that involve repetitive wrist extension and/or repetitive forearm supination/pronation with the elbow nearly fully extended. In patients with lateral epicondylitis, the pain can be exacerbated by resisted supination, resisted wrist extension, and tight grasping of objects. The most sensitive imaging finding is thickening and edema of the tendinous origin of the extensor carpi radialis brevis.

A 22-year-old man comes to the physician because of a progressive swelling and pain in his right ring finger for the past 2 days. The pain began while playing football, when his finger got caught in the jersey of another player who forcefully pulled away. Examination shows that the right ring finger is extended. There is pain and swelling at the distal interphalangeal joint. When the patient is asked to make a fist, his right ring finger does not flex at the distal interphalangeal joint. There is no joint laxity. Which of the following is the most likely diagnosis? This patient's injury is a common sports injury caused by forceful hyperextension of the DIP.

Rupture of the flexor digitorum profundus tendon at its point of insertion Rupture of the flexor digitorum profundus (FDP) tendon, also known as "jersey finger," is a common sports injury. The FDP, along with the flexor digitorum superficialis (FDS), is responsible for finger flexion. The tendons from the FDP muscle run through the carpal tunnel and insert on the palmar side of the distal phalanges of each finger, whereas the FDS tendons insert on the middle phalanx. This patient's inability to flex the DIP combined with the pain over the DIP joint, which is distal to the attachment of the FDS, indicate that he has ruptured his FDP tendon.

A 36-year-old woman comes to the emergency department because of left wrist pain and swelling that started immediately after a fall. She was walking her dog when the dog abruptly pulled away, causing her to fall forward onto both hands. Physical examination shows left wrist pain with pinching and grasping, moderate swelling, and mild tenderness. Her left thumb is placed in a thumb immobilization splint. An x-ray of the wrist 2 days later is shown. The most likely cause of these findings is a fracture of which of the following bones? This type of fracture is typically associated with tenderness to palpation of the anatomical snuffbox.

Scaphoid The scaphoid is the most commonly fractured carpal bone. Fracture results from an axial load injury onto a hyperextended and radially deviated wrist and manifests with mild wrist pain, swelling, and painful pinching and grasping, as seen in this patient. Although many scaphoid fractures can be treated conservatively with immobilization, proximal pole fractures carry a high risk of avascular necrosis and often require surgical fixation.

A 28-year-old man is brought to the emergency department 20 minutes after being involved in a bicycling accident. He complains of severe pain over the front of his right shoulder. He refuses to move his right arm. Physical examination shows supraclavicular swelling and bruising. The shoulder's range of motion is limited by pain. An x-ray of the shoulder shows a fracture of the middle third of the clavicle with complete superior displacement of the medial clavicular segment. Which of the following muscles is responsible for the displacement of this segment? The muscle in question is innervated by the spinal accessory nerve and functions predominantly in the rotation of the head.

Sternocleidomastoid The sternocleidomastoid muscle is superiorly attached to the mastoid process and inferiorly attached to the medial third of the clavicle. Therefore, in the setting of a fracture, as seen in this patient, the sternocleidomastoid pulls the medial segment of the fractured clavicle upwards.

An 18-year-old man comes to the physician because of severe left shoulder pain after a basketball match. During the game, the patient sustained an injury to the posterior part of his outstretched arm after being blocked by a defender. Examination shows no gross deformity of the left shoulder. Palpation of the shoulder elicits mild tenderness. Internal rotation of the arm against resistance shows weakness. These findings are most specific for injury to which of the following muscles? The affected muscle's function can also be tested with the lift-off test.

Subscapularis The subscapularis is a muscle of the rotator cuff and the main internal rotator of the shoulder. The muscle originates at the costal surface of the scapula and inserts into the lesser tubercle of the humerus. Innervation is provided by the lower and upper subscapular nerves. This patient's presentation suggests a traumatic subscapularis tendon tear, for which surgical repair is usually indicated.

A 34-year-old man is brought to the emergency department 3 hours after being bitten by a rattlesnake. He was hiking in the Arizona desert when he accidentally stepped on the snake and it bit his right leg. His pulse is 135/min and blood pressure is 104/81 mm Hg. Examination shows right lower leg swelling, ecchymosis, and blistering. Right ankle dorsiflexion elicits severe pain. A manometer inserted in the lateral compartment of the lower leg shows an intracompartmental pressure of 67 mm Hg. In addition to administration of the antivenom, the patient undergoes fasciotomy. Two weeks later, he reports difficulty in walking. Neurologic examination shows a loss of sensation over the lower part of the lateral side of the right leg and the dorsum of the right foot. Right foot eversion is 1/5. There is no weakness in dorsiflexion. Which of the following nerves is most likely injured in this patient? This patient's symptoms are most likely due to intraoperative injury of the nerve that innervates the fibularis longus and fibularis brevis muscles.

Superficial peroneal nerve Weakness with foot eversion and sensory loss to the dorsum of the foot (except the first digital webspace) and anterolateral leg indicate an injury of the superficial peroneal (fibular) nerve, which is located in the lateral compartment of the leg. This patient originally presented with features of compartment syndrome (severe leg swelling and pain, increased compartment pressure). Emergent fasciotomy is the treatment of choice in order to prevent vascular compromise and tissue ischemia; however, it is associated with a risk of nerve injury, as seen here. Risk factors for compartment syndrome include animal venoms (e.g., rattlesnake bites), circumferential burns, long bone fractures, and crush injuries.

A 17-year-old boy comes to the physician because of a 3-month history of pain in his right shoulder. He reports that he has stopped playing for his high school football team because of persistent difficulty lifting his right arm. Physical examination shows impaired active abduction of the right arm from 0 to 15 degrees. After passive abduction of the right arm to 15 degrees, the patient is able to raise his arm above his head. The dysfunctional muscle in this patient is most likely to be innervated by which of the following nerves? The findings in this patient are most likely caused by impairment of the supraspinatus muscle.

Suprascapular nerve The suprascapular nerve innervates the supraspinatus muscle and infraspinatus muscle, which are part of the rotator cuff. The supraspinatus muscle facilitates initial abduction of the arm. Impaired abduction from 0 to 15 degrees, which is seen in this patient, is a typical finding in supraspinatus injury.

A 52-year-old man comes to the physician because of right shoulder pain that began after he repainted his house 1 week ago. Physical examination shows right subacromial tenderness. The pain is reproduced when the patient is asked to abduct the shoulder against resistance with the arm flexed forward by 30° and the thumb pointing downwards. The tendon of which of the following muscles is most likely to be injured in this patient? Physical examination shows a positive empty can test, a maneuver that specifically assesses one of the rotator cuff muscles.

Supraspinatus Injury to the supraspinatus tendon and/or muscle manifests with pain triggered by overhead motion (e.g., when painting a house). Pain during the empty can test suggests injury to the supraspinatus tendon and/or muscle. In this test, forward flexion and internal rotation of the arm enables isolated examination of the supraspinatus muscle, independent of the deltoid muscle.

A 3629-g (8-lb) newborn is examined shortly after spontaneous vaginal delivery. She was delivered at 40 weeks' gestation and pregnancy was uncomplicated. Her mother is concerned because she is not moving her left arm as much as her right arm. Physical examination shows her left arm to be adducted and internally rotated, with the forearm extended and pronated, and the wrist flexed. The Moro reflex is present on the right side but absent on the left side. Which of the following brachial plexus structures is most likely injured in this infant? This infant's left arm is in a "waiter's tip" position, which suggests impaired shoulder abduction and lateral rotation, as well as impaired elbow flexion and forearm supination. This injury is common in neonates who require lateral neck traction during vaginal delivery and is associated with shoulder dystocia.

Upper trunk Erb palsy is due to injury to the upper trunk of the brachial plexus, which contributes to the motor and sensory function of the suprascapular nerve, musculocutaneous nerve, and axillary nerve. Erb palsy most commonly occurs secondary to shoulder dystocia or other birth trauma but can arise in adults as a result of neck and/or upper arm trauma.