Robert R. Slater, Jr.
DEFINITION
Disruption of the restraining structures on the volar surface of the joint between the metacarpal and proximal phalanx of the thumb may result in excessive joint motion and abnormal hyperextension.
Often painful, this instability frequently causes significant functional deficits because so much of what humans do with their hands depends on having a stable, pain-free thumb to oppose the other digits.
Acute injuries, including joint dislocations, must be treated correctly and promptly to afford the best chance for successful outcomes.
Chronic volar instability is seen less often than is collateral ligament incompetence, but it should not be overlooked. It can be treated effectively with a variety of techniques, which will be discussed in this chapter.
ANATOMY
The thumb metacarpophalangeal (MP) joint has features of both a ginglymus (hinge) joint and a condyloid joint. The joint moves mostly in a flexion–extension mode (ginglymus-style), but there are also elements of rotation and abduction–adduction in the normal joint (condyloid).
Thumb MP joint motion varies widely from individual to individual because of the spectrum of metacarpal head geometry seen in “normal” hands.
Some metacarpal heads are more rounded and allow greater flexion, extension, and rotation, while others are flatter and allow relatively less range of motion (ROM).
The joint derives its stability mostly from soft tissue constraints, not bony architecture (FIG 1).
The proper collateral ligaments originate from the region of the lateral condyles of the MC and pass palmarly and obliquely to insert on the palmar portion of the proximal phalanx.
The accessory collateral ligaments originate from the same region but slightly more proximal and traverse distally and palmarly in an oblique fashion to insert on the volar plate and sesamoids.
The volar plate serves as the floor of the MP joint. The adductor pollicis inserting into the ulnar styloid sesamoid at the distal edge of the volar plate and the insertions of the flexor pollicis brevis and abductor pollicis brevis into the radial sesamoid at the radial-distal edge of the volar plate provide additional volar support.
Those muscles also contribute fibers to the extensor mechanism by way of the adductor and abductor aponeuroses and thus provide a modicum of lateral joint stability.
FIG 1 • Anatomy of the thumb metacarpophalangeal joint.
Dorsally, the extensor pollicis brevis inserts onto the base of the thumb proximal phalanx and the extensor pollicis longus inserts at the base of the thumb distal phalanx; both traverse the MP joint and add to the stabilizing forces surrounding the joint.
The MP joint capsule itself surrounds the joint and contributes slightly to stability.
PATHOGENESIS
Dorsal dislocations of the thumb MP joint are much more common than are volar dislocations.4,5
The typical mechanism is a hyperextension force strong enough to rupture the volar plate and joint capsule.
For example, when a ball strikes a player's thumb or when there is a direct blow or fall that drives the phalanx into sudden hyperextension
Occasionally the radial or ulnar collateral ligaments (or both) of the MP joint are ruptured along with the volar plate. Their treatment is addressed in other chapters.
Sometimes the instability occurs in the setting of a patient with generalized ligamentous laxity such as Ehlers-Danlos syndrome (or other collagen disorders), but in those situations symptoms are less common and patients typically learn to compensate for the joint laxity.
NATURAL HISTORY
Posttraumatic instability left untreated may result in weakness of pinch and grip and progress to painful arthrosis due to the abnormal biomechanics of the damaged joint.
HISTORY AND PHYSICAL EXAMINATION
In traumatic cases it is important to inquire about the mechanism of injury.
If patients recall which way the thumb was “pointing” at the time of injury, it helps the examiner determine which structures were likely injured.
Was the joint dislocated and did it reduce spontaneously or with assistance from a coach, trainer, or the patient?
How difficult was the reduction?
Physical examination should include an assessment of ROM and grip and pinch strength, particularly in comparison with the contralateral thumb. Focal areas of tenderness should be ascertained. Residual tenderness along the volar plate may persist long after the injury.
The examiner should observe the resting joint posture; dislocated joints exhibit obvious deformity.
The examiner should check for open wounds and assess vascular status. Open wounds or vascular compromise mandate emergent treatment.
Limited or absent interphalangeal joint ROM suggests flexor pollicis longus tendon entrapment.
Dislocated or painful MP joints will have limited ROM.
Volar plate stability is assessed, since instability must be recognized and treated appropriately to maximize outcomes.
Severe collateral ligament injury is uncommon in conjunction with volar plate instability but must be recognized and treated where indicated.
An acute dislocation is rarely subtle, but when patients present with chronic instability symptoms, there may be guarding against full joint extension and soft tissue thickening in areas of chronic pathology.
IMAGING AND DIAGNOSTIC STUDIES
Plain radiographs of the thumb in three views (AP, lateral, and oblique) are requisite.
Injury films will reveal the direction of joint dislocation and any associated fractures (FIG 2A–C).
FIG 2 • (A) AP and (B) lateral injury films. (C) Oblique injury film of the thumb (vs. hand) show a dorsal metacarpophalangeal joint dislocation (arrows). D. Fluoroscopic imaging shows joint instability.
In the chronic setting, films may show evidence of prior fractures or bony injuries as well as the positions of the sesamoid bones relative to the joint space. In cases of chronic volar plate instability, the proximal phalanx may show subtle dorsal subluxation on the metacarpal head (that is more commonly noted when there has been injury to the dorsal capsule in association with collateral ligament damage).
Arthritic changes at the MP joint seen on the plain films will alter the treatment options. If the chronically unstable joint is already arthritic at the time of presentation, then an arthrodesis will be a better option than a soft tissue reconstruction.
Fluoroscopic real-time imaging and stress testing may confirm the suspected joint instability (FIG 2D).
A digital block may be placed to facilitate an adequate examination.
Ultrasound, MR imaging, and arthrography6–8 are advocated by some, but these studies are rarely of additional value in the assessment of thumb MP joint stability.
DIFFERENTIAL DIAGNOSIS
Fracture
Collateral ligament injury
Ligament laxity, generalized (eg, Ehlers-Danlos syndrome)
Arthritis
Locked trigger thumb (stenosing tenosynovitis)
NONOPERATIVE MANAGEMENT
Most acute thumb MP dislocations can be reduced closed.
The reduction maneuver for a dorsal dislocation involves very slight hyperextension of the MP joint followed by direct volarly directed pressure on the base of the proximal phalanx to gently slide the phalanx over the metacarpal head and back into proper alignment.
Subtle, slight rotation (pronation and supination) at the same time may help ease any interposed soft tissue out of the way as the phalanx reduces.
Longitudinal traction and excessive hyperextension should be avoided because the soft tissue tension generated may cause one or more structures surrounding the joint to slip between the metacarpal head and the proximal phalanx, blocking reduction.
The volar plate, the flexor pollicis longus, and one or both sesamoid bones have all been described as culprits that have become incarcerated in the joint space, preventing reduction.2,3,5,9
Once the joint is reduced, the patient should be able to flex and extend the thumb joints and the radiographs should show concentrically reduced and congruent joint surfaces (FIG 3).
If either of those conditions is not met, it suggests there may be residual soft tissue interposed in the joint, an indication for open reduction.
After successful closed reduction, the thumb should be splinted in flexion to relax the injured volar structures.
After a few days, when the acute swelling has dissipated, the splint may be changed to a thumb spica cast, again with the MP joint flexed, for an additional 2 to 3 weeks. Rehabilitation can commence thereafter, emphasizing ROM exercises within a safe zone of motion from full flexion to just short of neutral extension and then increasing gradually to unrestricted ROM and hand use by 6 weeks after injury.
For patients engaged in activities that risk forced hyperextension of the thumb, taping or splinting may be required during these endeavors for an additional period.
Failure to recognize or treat the acute instability or overly aggressive progression to full, unlimited hand use may result in chronic volar instability.
FIG 3 • Postreduction views of the patient in Figure 2 confirm congruent joint surfaces and satisfactory alignment, including the sesamoid bone positions.
Nonoperative management then is limited to providing the patient with a custom-molded splint to prevent hyperextension of the MP joint.
A properly trained hand therapist is invaluable in assisting patients who prefer to manage the problem nonoperatively and use a protective splint rather than proceeding with surgical treatment for their chronic instability.
Volar dislocation is very rare. There are only a few cases reported in the literature, and all required open reduction.4,5
SURGICAL MANAGEMENT
Open reduction is required when attempts at closed manipulation and reduction of acute dislocations fail.
Failure is typically the result of soft tissue interposition in the joint that blocks reduction. Tissue interposition may have happened at the time of the original injury or as a result of a well-meaning coach, friend, or medical colleague trying to reduce the dislocation by applying vigorous traction, which can cause the soft tissues to become incarcerated.
Chronic instability, which is persistently symptomatic despite nonoperative treatment, is best treated with a soft tissue stabilization technique unless moderate to severe arthrosis is present or the instability is global and exceedingly severe. In those cases, arthrodesis is the treatment of choice.
Preoperative Planning
The physician should review all imaging studies. In most cases, those will be limited to plain radiographs and perhaps spot films from fluoroscopic evaluations.
Films should be reviewed for any bony abnormalities, especially nondisplaced fractures. One should avoid fracture displacement during intraoperative manipulation of the thumb.
In fracture-dislocations of the MP joint, larger fragments are stabilized using Kirschner wires or screws and smaller avulsion-type fragments are excised and the ligament is secured to the bone.
For chronic cases, it is important to review the films and rule out osteoarthrosis, which would warrant different treatment strategies.
Examination under anesthesia with the assistance of fluoroscopy can be useful to confirm the degree and direction of joint instability.
Spot films obtained before and after surgical stabilization can be helpful visual aids for use in postoperative discussions with the patient and his or her family to explain again the nature of the problem and how it was treated.
Positioning
The patient should be supine on the operating table with a standard hand table attached and projecting out from the operating table to support the operative hand.
A tourniquet should be placed on the operative arm and checked for proper function and pressure before initiation of the surgery (typically 250 mm Hg or 100 mm Hg greater than systolic blood pressure).
Approach
Acute, irreducible MP joint dislocations are best approached from the volar side of the joint so that any soft tissues that may be trapped in the joint can be identified and carefully protected, before they are injured by approaching them “blindly” from the dorsal side!
That assumes there are no open wounds, which then would alter one's approach accordingly by incorporating the traumatic wound into the surgical incision.
A lateral approach is also possible, but less often used.
Chronic MP joint volar instability that is amenable to soft tissue stabilization should also be approached from the volar aspect of the joint so the pathology can be visualized and addressed directly.
If chronic MP joint instability has resulted in arthritis, arthrodesis is a better solution. This may be accomplished in a variety of ways using a variety of hardware options, including screws, plates, and wires. All are best placed through a dorsal approach.
TECHNIQUES
OPEN REDUCTION OF ACUTE MP JOINT DISLOCATIONS
Make a zigzag (Bruner-type) incision, centered at the level of the MP joint.
Gently elevate the skin flaps to expose the underlying soft tissues, which will by definition be displaced from their usual locations (TECH FIG 1).
Identify the neurovascular bundles and mobilize them enough to ensure their protection.
Small rubber loops can be placed around them if desired both for protection and for easy identification.
Retract the soft tissues enough to identify whatever structure is interposed into the MP joint and reflect it out of the joint.
Most often that is the flexor pollicis longus tendon or the volar plate. The volar plate may have the sesamoids still attached to its distal edge if it has failed proximally.
Reduce the joint and check for smooth, congruent joint motion.
Check the stability of the collateral ligaments. Providing the joint does not gap open more than 25 degrees due to collateral ligament damage (rare), no further treatment for that component of the joint injury is needed.
TECH FIG 1 • Open reduction of dorsal thumb metacarpophalangeal joint dislocation. A. Make a volar surgical approach and carefully identify the neurovascular bundles; tag and protect them with soft rubber loops (*). B.The flexor pollicis longus (solid arrow) is interposed and trapped behind the metacarpal head (open arrow). C. Umbilical tape (arrow) passed around the flexor pollicis longus delivers the tendon safely out of harm's way, allowing the joint to be reduced. D. Intraoperative fluoroscopy is helpful for verifying anatomic reduction (arrow) with congruent joint surfaces and smooth gliding through a safe range of motion.
In some cases it may be possible to place sutures through the volar plate at its torn proximal or distal edge and tack that back to its normal insertion point. Otherwise, simply replacing the plate in normal alignment will be adequate when combined with proper rehabilitation (discussed later in the chapter).
Replace the neurovascular bundles and flexor pollicis longus into their proper locations and close the wound in routine fashion.
CHRONIC VOLAR INSTABILITY
Volar Plate Advancement and Sesamoid Arthrodesis
The Tonkin procedure11 was originally described to treat MP instability resulting secondarily from osteoarthrosis of the thumb carpometacarpal joint or in patients with cerebral palsy, but it is now considered a valuable technique or treating posttraumatic instability of the MP joint.
Approach the MP joint through a volar or volar-radial incision (TECH FIG 2A).
Divide the accessory radial collateral ligament at its insertion into the volar plate and mobilize the plate to advance it proximally (TECH FIG 2B).
Denude the articular surface of the sesamoid bones. A Beaver blade works well.
Decorticate a trough along the retrocondylar fossa of the metacarpal neck to accept the sesamoids.
Drill Keith needles through that area of the retrocondylar fossa using a wire driver. The needles should exit the metacarpal dorsally (TECH FIG 2C).
Advance the sesamoids and volar plate into the prepared trough and secure the sutures dorsally (TECH FIG 2D).
Variations:
Schuurman and Bos10: Place sutures through the proximal edge of the volar plate and pass them through the metacarpal via the Keith needles. Reinforce the construct with nonabsorbable sutures to local tissue where possible.
TECH FIG 2 • Sesamoid arthrodesis in the manner of Tonkin. A,B. Through a volar or radial lateral approach, create a cortical defect in the metacarpal retrocondylar fossa (solid arrow), which will accept the denuded sesamoids while preserving the articular surface of the metacarpal head (dashed arrow). C. Advance and secure the volar construct including the sesamoids into the prepared trough (arrow), using sutures in the volar plate that are brought through the metacarpal neck via Keith needles drilled through the bone. D. The sutures are tensioned and tied over the dorsal metacarpal (*). E. A Kirschner wire is drilled across the metacarpophalangeal joint to keep it flexed 30 degrees, protecting the repaired volar structures during initial healing.
Eaton and Floyd1: Place sutures in the proximal corner of the volar plate and pass them subperiosteally from volar to dorsal around the metacarpal and secure them to advance the volar plate snugly into the prepared retrocondylar fossa.
Pin the MP joint in about 30 degrees of flexion using a Kirschner wire (TECH FIG 2E).
Close the wound in routine fashion and apply dressings and a thumb spica splint.
Remove the sutures at 10 to 14 days as usual and apply a thumb spica cast.
Remove the Kirschner wire and cast at 4 to 6 weeks and begin active flexion exercises.
Tendon Graft Tenodesis
A procedure used by Littler and cited by Glickel et al3 has been described whereby a free tendon graft (usually the palmaris longus) is woven through drill holes in the proximal phalanx and metacarpal and secured in place to provide a passive restraint against MP joint hyperextension.
However, the bulk of soft tissue that results from this procedure and the amount of dissection needed to perform it have caused this operation to fall out of favor.
Local tissue mobilization in conjunction with suture anchors provides a better solution.
ARTHRODESIS
Cannulated Headless Compression Screw Fixation
Make a dorsal longitudinal approach (TECH FIG 3A–D).
Split the interval between extensor pollicis longus and brevis.
Split the joint capsule longitudinally.
Mobilize the joint adequately. That requires releasing the remaining collateral ligaments and recessing the volar plate enough to deliver the metacarpal head fully into view.
Use a water-cooled oscillating saw to remove the metacarpal head. Angle the cut from dorsal-distal to palmar-proximal so that the final positioning will leave the MP joint surface flexed about 15 degrees (TECH FIG 3E).
TECH FIG 3 • Clinical examination (A) shows metacarpophalangeal joint hyperextension (arrow), but in this case it is chronic and associated with joint space narrowing and asymmetry indicative of osteoarthrosis, as seen on radiographs (B,C), as well as hyperextension (arrow in B). D. Through a dorsal approach, the interval between the extensor pollicis brevis and longus (arrow) is developed and the joint is entered. E. Remove the metacarpal head with an oscillating saw. F. Prepare the base of the proximal phalanx with a burr. G. The opposing bone surfaces should be flush-cut and angled slightly so the final arthrodesis position is 15 to 20 degrees of flexion. H.The guidewire for the chosen cannulated screw is drilled across from metacarpal into the medullary canal of the proximal phalanx. I. Its position is checked with fluoroscopy. J. After the proper implant length is measured and the leading cortex overdrilled, the screw is inserted over the guidewire. K. Confirm correct final positioning with fluoroscopy. L.The joint capsule is closed and the extensor mechanism reapproximated.
All the flexion for the arthrodesis will be accomplished by this cut. Preparation of the proximal phalanx base will be perpendicular to the longitudinal axis of that bone and will not add flexion.
Use a burr to prepare the base of the proximal phalanx, removing any remaining articular cartilage and eburnated subchondral bone (TECH FIG 3F). Osteophytes and ridges should be trimmed away also to make a flush surface that will oppose the MC surface (TECH FIG 3G).
Alternatively, carefully protect the underlying flexor pollicis longus and use a water-cooled oscillating saw to remove the needed bone.
Avoid excessive bone resection that will shorten the thumb. Cup and cone reamers are an alternative to straight bone cuts and may minimize shortening and maximize flexibility in positioning the arthrodesis.
Reduce the fusion surfaces and drive a guidewire for the selected cannulated screw set from the metacarpal into the medullary cavity of the proximal phalanx (TECH FIG 3H).
Be certain that the starting point for the guidewire is sufficiently proximal on the dorsal surface of the metacarpal that the screw does not fracture the cortical bridge when inserted.
Alternatively, the guidewire may be drilled in a retrograde fashion starting at the cut end of the metacarpal head. The fusion surfaces are then reduced and the guidewire is advanced into the phalanx in an antegrade manner.
Consider placing a second temporary Kirschner wire to increase stability and minimize rotation during screw insertion.
Confirm that the overall alignment is satisfactory radiographically and clinically (TECH FIG 3I).
The metacarpal and phalanx should be co-linear in the AP plane and flexed about 15 to 20 degrees in the lateral plane. Rotation should be neutral or slight pronation for pinch.
Intraoperative fluoroscopy is helpful to confirm correct alignment.
Adjust the position of the guidewire for the cannulated screw such that its distal tip is just past the narrowest portion of the proximal phalanx. A screw ending at this level will experience excellent purchase and gain maximum stability. Measure the Kirschner wire length, choose the screw (keeping in mind the likelihood of compression at the arthrodesis site), and then advance the guidewire distally into the cortex.
Drill, tap, and place the selected screw, avoiding any prominence over the dorsal metacarpal. Tighten securely while the reduction is compressed manually (TECH FIG 3J).
Reconfirm correct alignment in all planes, paying particular attention to rotation. Confirm satisfactory hardware positioning (TECH FIG 3K).
Morselized bone graft can be harvested from the resected metacarpal head and packed in and around the arthrodesis site if needed.
Close the joint capsule with absorbable suture to minimize extensor tendon adhesions.
Approximate the extensor tendon interval with interrupted, inverted permanent suture and close the wound in a routine fashion (TECH FIG 3L).
Place a forearm-based thumb spica splint.
Plate and Screw Fixation
It may be desirable to use plate and screw fixation rather than cannulated screws in such cases as nonunion after attempted arthrodesis, failure of implant arthroplasty, and traumatic injuries with severe deformity, bone loss, or segmental defects (TECH FIG 4).
The advantage of plates and screws is that more rigid, secure fixation can be achieved immediately, avoiding the concern for rotation or loosening around a single cannulated screw.
The disadvantage of that technique is that hardware prominence and tendon irritation and adhesions are more often a subsequent source of trouble.
If plate and screw fixation is chosen as the desired technique, then the overall approach and bone preparation are similar to that described for the cannulated screw technique.
With the arthrodesis site reduced and temporarily stabilized with a Kirschner wire, a 2.0-mm, five-hole compression plate is contoured to the dorsal surface of the bones.
The plate is first secured distally and then applied proximally using compression technique principles.
It is critical to avoid long screws and irritation of the flexor pollicis longus.
Closure and postoperative care are similar to that described earlier.
TECH FIG 4 • Arthrodesis with plate and screw fixation.
POSTOPERATIVE CARE
Acute Dislocation
The MP joint is generally stable once a dislocation is reduced acutely, whether by closed or open methods.
The MP joint should be held in about 30 degrees of flexion in a short-arm thumb spica splint or cast for 2 weeks.
ROM exercises can begin thereafter, using a removable thumb spica splint for an additional 4 weeks, gradually weaning out of the splint and advancing activities as symptoms allow.
Supervised hand therapy is often helpful for patients to guide their recovery of motion and strength and optimize their outcomes.
Chronic Instability
Volar plate advancement procedures and the Tonkin sesamoid arthrodesis procedure should be protected with the thumb MP joint flexed in a cast for 4 to 6 weeks, depending on the surgeon's assessment of tissue quality and patient compliance. Then supervised ROM can begin, but MP joint hyperextension forces should be avoided for 8 to 12 weeks.
MP joint arthrodesis procedures require longer protection so that the fusion site is not stressed or disrupted before final bony union.
Generally it is best to use a thumb spica splint (plaster) for the first 10 to 14 days after surgery until the swelling decreases and the sutures are removed.
Then a thumb spica cast can be used for an additional 3.5 to 4 weeks, at which time a custom-molded, removable splint can be used to protect the arthrodesis site but allow ROM of the uninvolved adjacent joints of the hand to prevent undue stiffness.
By 12 weeks most arthrodeses are healed solidly enough to allow unrestricted hand use.
OUTCOMES
Acute volar instability and dorsal dislocations of the MP joint that are treated appropriately can be expected to have a good prognosis.2,3,9 Whether the dislocation is reduced closed or open reduction is required, once the joint is reduced it is usually stable.
Following rehabilitation as outlined above, there well may be some residual joint stiffness. While that may continue to improve for up to 1 year after injury, the lost range of motion is rarely a functional problem.
Tonkin et al11 reported successful outcomes in 38 of 42 cases (90%) of sesamoid arthrodeses for chronic MP joint volar instability. Those results compare favorably with outcomes following other capsulodesis and volar plate reinforcing procedures. The advantage of all such procedures is that hyperextension is blocked, restoring stability to the joint, while still allowing the MP joint to flex. Mean loss of flexion compared with the preoperative condition was 8 degrees in the series reported by Tonkin et al.11
Outcomes after arthrodesis for chronic volar instability must be viewed with the proper surgical goal in mind. The goal is to relieve pain (from instability and arthritic change) and provide stability to the thumb ray. Success rates are high, barring any unfortunate complications as discussed next.
COMPLICATIONS
Complications following MP joint dislocations are uncommon and mostly limited to the sequelae of concomitant soft tissue injuries.
Damage to the adjacent neurovascular structures can result from the initial traumatic injury or careless surgical technique at the time of open reduction.
Damage to the flexor pollicis longus tendon may occur when it gets trapped in the joint or again when it is manipulated surgically during an open reduction.
A complication encountered more often is persistent, chronic instability of the MP joint that results from failure to recognize the nature of the original injury or rehabilitate it properly.
Complications after treatment for chronic MP joint volar instability are likewise uncommon and generally related to failure of the chosen procedure.
Volar plate advancement can fail due to stretching out over time or a second trauma that causes acute rupture of the repair or sutures.
Nonunion of attempted arthrodesis is always a risk, but fortunately in the small joints of the hand, including thumb MP joints, it is uncommon. Nonunion rates range from 0% to 12% in several reported series.12
Hardware causing soft tissue irritation is a potential complication. That can be from superficial pin tract infections in cases where Kirschner wires are used to maintain joint reduction to extensor tendon irritation when fusions are done using plates and screws.
REFERENCES
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10. Schuurman AH, Bos KE. Treatment of volar instability of the metacarpophalangeal joint of the thumb by volar capsulodesis. J Hand Surg Br 1993;18B:346–349.
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12. Weiland AJ. Small joint arthrodesis. In Green DP, Hotchkiss RN, Pederson WC, eds. Green's Operative Hand Surgery. Philadelphia: Churchill Livingstone, 1999:95–107.