Here are high-level, case-based MCQs on carpal instability for a PhD-level orthopedic exam, complete with answers and explanations.
Case-Based MCQs on Carpal Instability
Case 1: Role of the Lunate in Carpal Stability
A 35-year-old construction worker presents with progressive wrist pain after a fall. Examination reveals pain on the dorsal aspect of the wrist and decreased grip strength. X-ray shows a widened scapholunate interval. What is the primary role of the lunate in carpal stability?
A) It connects to the scaphoid and triquetrum.
B) It forms the primary connection to the radius.
C) It stabilizes the pisiform bone.
D) It has no significant role in stability.
E) It primarily supports the capitate.
Answer: A) It connects to the scaphoid and triquetrum.
Explanation:
The lunate plays a central role in wrist stability by forming a link between the scaphoid and triquetrum. It allows for coordinated movement of the proximal carpal row. Disruptions in these connections, such as scapholunate dissociation, lead to instability and dysfunction.
Case 2: Scapholunate Dissociation
A 29-year-old tennis player presents with persistent dorsal wrist pain and a sensation of weakness when gripping. A “Terry Thomas sign” is noted on X-ray. Which type of carpal instability is most likely present?
A) VISI (Volar Intercalated Segment Instability)
B) CID (Carpal Instability Dissociative)
C) DISI (Dorsal Intercalated Segment Instability)
D) CIND (Carpal Instability Nondissociative)
E) Adaptive instability
Answer: C) DISI (Dorsal Intercalated Segment Instability)
Explanation:
DISI occurs due to scapholunate ligament rupture, causing the scaphoid to flex and the lunate to dorsiflex. The hallmark finding on X-ray is a scapholunate gap >3 mm (“Terry Thomas sign”) and an increased scapholunate angle >60°.
Case 3: Radiographic Findings in Scapholunate Dissociation
A 40-year-old office worker presents with chronic wrist pain aggravated by typing. X-ray shows an increased scapholunate interval of 4.5 mm but normal alignment of other carpal bones. What is the most likely diagnosis?
A) Lunotriquetral dissociation
B) Scapholunate dissociation
C) Radial deviation
D) Increased carpal arch
E) Ulnar translocation
Answer: B) Scapholunate dissociation
Explanation:
A scapholunate gap >3 mm is diagnostic of scapholunate dissociation. This condition leads to DISI deformity if untreated. The Watson test (scaphoid shift test) is positive, and MRI can confirm ligamentous injury.
Case 4: Ligament Injury Leading to DISI
A 42-year-old weightlifter complains of radial-sided wrist pain with difficulty gripping. MRI reveals a complete tear of a key ligament, leading to DISI deformity. Which ligament is most likely injured?
A) Lunotriquetral ligament
B) Scapholunate ligament
C) Radial collateral ligament
D) Pisohamate ligament
E) Dorsal radiocarpal ligament
Answer: B) Scapholunate ligament
Explanation:
Scapholunate ligament injury is the most common cause of DISI, leading to scaphoid flexion and lunate dorsiflexion. This results in instability and arthritis if left untreated.
Case 5: Normal Scapholunate Angle
A 28-year-old athlete undergoes wrist imaging following a fall. The scapholunate angle on X-ray is measured at 45°. How should this finding be interpreted?
A) Normal
B) Suggestive of VISI
C) Suggestive of DISI
D) Indicative of midcarpal instability
E) Consistent with scaphoid fracture
Answer: A) Normal
Explanation:
The normal scapholunate angle is 30–60°.
- >70° suggests DISI (dorsal tilt of lunate).
- <30° suggests VISI (volar tilt of lunate).
Case 6: Associated Conditions in Scapholunate Dissociation
A 50-year-old patient with untreated chronic scapholunate dissociation presents with worsening wrist pain. X-ray shows radiocarpal and midcarpal arthritis. What is the most common associated condition?
A) Kienböck’s disease
B) Osteoarthritis
C) Rheumatoid arthritis
D) TFCC tears
E) Carpal tunnel syndrome
Answer: A) Kienböck’s disease
Explanation:
Chronic scapholunate dissociation leads to progressive degeneration of the lunate, predisposing the patient to Kienböck’s disease (avascular necrosis of the lunate).
Case 7: Surgical Management of Chronic Scapholunate Dissociation
A 38-year-old mechanic presents with persistent scapholunate dissociation for 8 weeks despite conservative treatment. MRI confirms ligament rupture. What is the most appropriate surgical intervention?
A) Closed reduction
B) Dorsal capsulodesis
C) Scaphoidectomy
D) Ulnar nerve decompression
E) TFCC reconstruction
Answer: B) Dorsal capsulodesis
Explanation:
Dorsal capsulodesis is indicated in chronic scapholunate dissociation to stabilize the scaphoid and prevent further carpal collapse. Early-stage injuries (<6 weeks) may be treated with ligament repair, while late-stage cases with arthritis require scaphoidectomy or wrist fusion.
Case 8: MRI Findings in Scapholunate Dissociation
A 30-year-old gymnast presents with recurrent wrist pain. MRI shows a complete tear of the scapholunate ligament without significant arthritic changes. What is the most specific MRI finding?
A) Increased joint fluid
B) Scapholunate ligament tear
C) Ulnar variance
D) Dorsal wrist edema
E) Midcarpal instability
Answer: B) Scapholunate ligament tear
Explanation:
MRI is the gold standard for soft tissue evaluation and can detect partial or complete tears of the scapholunate ligament, which confirms scapholunate dissociation.
Case 9: Type of Carpal Instability
A 33-year-old weightlifter experiences chronic wrist pain. MRI reveals abnormal motion within the same row of carpal bones, but no complete ligament rupture. What type of carpal instability is present?
A) Nondissociative instability
B) Complex instability
C) Dissociative instability
D) Adaptive instability
E) Midcarpal instability
Answer: C) Dissociative instability
Explanation:
Dissociative instability occurs when there is abnormal movement within the same carpal row (e.g., scapholunate or lunotriquetral dissociation). It differs from CIND (carpal instability nondissociative), which affects the entire row.
Case 10: Clinical Presentation of Scapholunate Dissociation
A 37-year-old golfer presents with radial wrist pain and a clicking sensation when swinging a club. Watson’s test is positive. What is the most likely diagnosis?
A) Perilunate dislocation
B) Scapholunate dissociation
C) TFCC tear
D) Kienböck’s disease
E) Midcarpal instability
Answer: B) Scapholunate dissociation
Explanation:
A positive Watson’s test (painful scaphoid subluxation on radial deviation) is characteristic of scapholunate dissociation, leading to wrist pain and instability.
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