Thumb MP Joint

THUMB MP JOINT

Anatomy

The thumb MP joint allows flexion, extension, abduction, adduction, and a limited range of rotation. Flexion is to 115o, abduction/adduction of about 10o. The ulnar aspect has particular significance, with the adductor aponeurosis, an ulnar expansion of the dorsal aponeurosis, supporting the ulnar collateral ligament dynamically.

Ulnar Collateral Ligament Injury (Gamekeeper’s/Skier’s Thumb)

The mechanism is sudden abduction/extension, resulting in injury to the ulnar collateral ligament and volar plate.
The “Stener” lesion occurs when the completely torn ligament folds back upon itself and is held in this retracted position by the adductor aponeurosis, leaving no chance for repair. The prevalence of the Stener lesion is reportedly variable, but probably occurs in 50%. It is unlikely that you will accurately be able to tell on physical exam whether or not it is present.
The clinical diagnosis of complete tear is made by stress testing under some form of regional anesthesia (median and radial nerve block). What constitutes a positive test is controversial – either 10o more than the other side, or 35o.
The radiographic diagnosis of complete tear is by stress testing as well. Some people argue that you might create a Stener lesion by doing this. Note any chip or avulsion fractures off the base of the proximal phalanx.

Treatment – Acute Partial Tears
Thumb spica cast with MP slightly flexed for 3 to 4 weeks.

Treatment – Acute Complete Tears
Most commonly, the ligament is torn away from the proximal phalanx. Operative repair with pull-out suture over a button, or bone anchor.
If midsubstance tear, direct operative repair.
Also, operate on those with a displaced intra-articular fracture involving 25% or more, or a small avulsion fracture that is displaced more than 5 mm.

Treatment – Chronic Partial Tears
Cast or splint immobilization for 3 weeks, then intense physio.

Treatment – Chronic Complete Tears
If joint intact – modified Neviaser reconstruction
If joint arthritic – fuse.

Radial Collateral Ligament Injuries

These are less common than the ulnar collateral injuries, and theoretically do not have the same anatomic obstacles to healing.

Treatment
– immobilization in thumb spica for 3-4 weeks.
– a similar Neviaser-like reconstruction has been described as well.

Dislocations of the Thumb MP Joint

Dorsal dislocations are more common than volar, and may be irreducible owing to the volar plate (and less commonly, FDP).
The diagnosis of irreducibility, like in the finger MPs, should be made clinically by the parallel alignment of the proximal phalanx and a dimple in the thenar eminence. Also, on x-ray, the interposition of the sesamoid within the joint is pathognomonic of a complex dislocation.

Treatment
Closed reduction should be done with the wrist flexed (to relax the FDP), hyperextension of the MP with axial load so as to not entrap the volar plate, then flexion. If successfully reduced, check the lateral stability of the joint, and if okay, then mobilize immediately.
If irreducible, open. Early motion should be instituted postoperatively.

Volar Plate Injuries

These are caused by hyperextension of the thumb, and are diagnosed by demonstrating increased hyperextension compared to normal side, or by volar pain on hyperextension.

Treatment
Thumb spica immobilization for 3-4 weeks with the MP in 15o-20o of flexion.
For chronic injuries, a volar plate capsulodesis has been described – rarely done.

Sesamoid Fractures

Rare. Most people have 5 sesamoids – one at the thumb IP joint, two at the thumb MP joint, and one each at the MP joints of the index and little fingers. The thumb MP joint sesamoids are universally present, and they reside in the volar plate. The tendon of adductor pollicis inserts into the ulnar sesamoid, and the tendon of flexor pollicis brevis into the radial sesamoid.

Treatment
Thumb spica immobilization for 3-5 weeks.
Excision if symptomatic.

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