Chapter 07: Wrist

6. The following questions are based on the data presented in Figure 7-24. (A) Which muscle would produce the greatest flexion torque at the wrist, the flexor carpi radialis or the flexor digitorum superficialis? (B) Which muscle has the longest moment arm for ulnar deviation torque? (C) Which muscle is the most direct antagonist to the flexor carpi ulnaris?

(A) The data in Figure 7-24 allow you to estimate a muscle's relative force production (based on physiologic cross-sectional area) and its internal moment arm relative to the capitate. The product of these variables allows a relative estimate of torque production across the wrist. Visual analysis of the data clearly shows that the flexor digitorum superficialis would be able to produce greater flexor torque at the wrist than the flexor carpi radialis. (B) The extensor carpi ulnaris has the longest moment arm for this torque. (C) The extensor pollicis longus or the extensor carpi radialis brevis is the most direct antagonist.

3. Assume that trauma associated with a fractured distal radius created a permanent 25-degree dorsal tilt of the distal radius (review Figure 7-4, B). What are some probable functional impairments that may result from this malalignment?

A 25-degree dorsal tilt of the distal radius would create morphologic incongruity within the surfaces of the radiocarpal and distal radio-ulnar joints. This may result in reduced wrist motion (most likely in the direction of flexion) and reduced pronation and supination of the forearm. Because of increased joint stress, degenerative arthritis may eventually develop.

9. How would you maximally stretch the extensor carpi radialis longus muscle?

As with any muscle, maximal elongation (stretch) occurs by moving the underlying joints in positions opposite to the muscle's actions. Maximally stretching the extensor carpi radialis longus occurs through a combination or wrist flexion and ulnar deviation, along with pronation of the forearm.

5. Justify the importance of the capitate bone with regard to the osteokinematics of the entire wrist and hand.

Because of the relatively firm articulation between the capitate and the base of the third metacarpal, movement of the capitate directs the path of movement of the entire long axis of the wrist-and-hand. Furthermore, on average, both axes of rotation for movement of the wrist pass through (or near) the capitate bone. These concepts provide a useful reference when measuring the range of motion and torque produced across the wrist.

15. Describe the muscular interaction between the flexor carpi ulnaris and flexor carpi radialis during active flexion of the wrist.

Both muscles produce flexion at the wrist; however, each muscle neutralizes the other's frontal plane action.

4. Describe the arthrokinematic pattern for flexion and extension at the radiocarpal joint.

During flexion the proximal row of carpal bones roll palmarly and slide dorsally; the capitate (a representative of the medial compartment of midcarpal joint) rolls palmarly and slides dorsally. During extension the proximal row of carpal bones roll dorsally and slide palmarly; the capitate rolls dorsally and slides palmarly. As a guiding principle, a bone (or set of bones) rolls in the same direction as the overall segment's osteokinematics.

13. Compare the convex-concave joint relationships that exist within the medial and lateral compartments of the midcarpal joint of the wrist. Describe how these relationships affect the arthrokinematics of the joint during flexion and extension.

Medial compartment of midcarpal joint: A convex (distal) segment articulates with a concave (proximal) segment. Assuming distal-on-proximal segment (sagittal plane) osteokinematics, the arthrokinematics occur as a roll-and-slide in opposite directions. Lateral compartment of midcarpal joint: A concave (distal) segment articulates with a convex (proximal) segment. Assuming distal-on-proximal segment (sagittal plane) osteokinematics, the arthrokinematics occurs as a roll-and-slide in similar directions.

7. Which two tendons of the thumb share the same fibrous tunnel within the extensor reticulum of the wrist?

The extensor pollicis brevis and abductor pollicis longus share the same tunnel.

8. What is the role of the scaphoid in providing mechanical stability to the lunate?

The lunate is the most naturally unstable of the carpal bones, primarily because of its shape and lack of muscular attachments. The lunate receives much of its mechanical stability through its attachments to the scaphoid, primarily through the scapholunate ligament. The scaphoid is able to provide this stable base because of its firm attachment to the stable distal row of carpal bones, specifically the trapezoid and trapezium. Fractures of the scaphoid or ruptures of either the scaphotrapezial or the scapholunate ligament can therefore mechanically disrupt the stability of the lunate.

10. Which extrinsic ligaments naturally resist an ulnar translocation of the carpus?

The palmar and dorsal radiocarpal ligaments have a favorable line of force to resist ulnar translation of the carpus relative to the radius.

12. Which carpal bones normally do not contact the capitate bone?

The pisiform, triquetrum, and trapezium usually do not contact the capitate.

11. A patient had severe trauma to the proximal radius and adjacent interosseous membrane that required a partial resection of the radial head. Describe possible functional impairments or pathologies that might result from a subsequent 6- to 7-mm proximal migration of the radius.

The radius may migrate proximally relative to the ulna, especially if the radial head has been surgically removed. This migration is more likely if the interosseous membrane is torn. Proximal migration of the radius may cause a "positive ulnar variance" at the wrist. A positive ulnar variance of more than a few millimeters can lead to a potentially painful and disabling "ulnar impaction syndrome." Increased stress placed on tissues such as the triangular fibrocartilage, triquetrum, lunate, and scapholunate ligament can create pain and limitation of movement at the wrist and forearm. In extreme cases the stress can lead to painful osteoarthritis.

1. How does the tendon of the flexor carpi radialis reach the base of the metacarpal bones without actually entering the carpal tunnel?

The tendon of the flexor carpi radialis courses distally across the wrist through a narrow tunnel located in the palmar surface of the transverse carpal ligament. Technically, therefore, the tendon does not course within the carpal tunnel.

2. Cite factors that justify the greater range of ulnar deviation as compared with radial deviation of the wrist.

Ulnar deviation range of motion exceeds radial deviation of the wrist because (1) the ulnocarpal space offers little resistance to the path of ulnar deviation, and (2) the distally projecting styloid process of the radius blocks the extremes of radial deviation.

14. List all muscles that have a full or partial proximal attachment to the lateral epicondyle of the humerus. Which nerve innervates all these muscles?

• Brachioradialis • Extensor carpi radialis longus • Extensor carpi radialis brevis • Extensor carpi ulnaris • Extensor digitorum • Anconeus • Supinator All these muscles are innervated by the radial nerve.