For 1st Metatarsophalangeal Joint Arthroplasty
 utilizing the Biomet Merck Ltd. Total Toe System

There is a forty five year history of invention and technique for attempt at successful artificial joint replacement of the diseased articular surfaces of the 1st metatarsophalangeal joint or great toe joint. The persistence of inventor-surgeons attests to the significance attributed to reconstruction of the great toe joint.  The goal has been and remains, to eliminate pain and/or painful loss of motion and restore predictable function of this small yet vitally necessary joint. The accomplishment of this goal has required time and technological advancement. Now that science and bioengineering have made possible understanding, shapes and materials, it is incumbent upon the surgeon and therapist to appreciate why this effort has been made and how to apply this technology. The mechanics of quality motion at the 1st metatarsophalangeal joint will be discussed. A rationale for reconstruction of this joint, rather than the more traditional techniques employing joint destructive procedure will be advanced. It is the intention of this author to bring the 1st metatarsophalangeal joint that has been affected by degenerative arthrosis at the same rate as larger reconstructed joints, into the  light. 

The etiology of hallux limitus/rigidus, not withstanding direct trauma to the joint, has been debated for several decades. The controversy may well be within the scope of the ongoing chicken or the egg  argument. To keep this subject in perspective, hallux limitus/rigidus is a clinical presentation of underlying degenerative joint disease. DJD of the 1st metatarsophalangeal joint can also manifest with severe hallux abducto valgus. The joint will be painful and inflammed but motion may not be restricted because the joint is subluxed and moves in an independant range of motion, albeit abnormal and grating. To restore normal parameters of congruency to such a joint, would be to ignore a major component of the deformity. Metabolic illness, too, manifests in the 1st metatarsophalangeal joint as degenerative joint disease (e.g.: gouty arthritis, rheumatoid arthritis and psoriatic arthritis.) The disease entity affecting the great toe joint may vary, but the effect  remains the same; pain and loss of function.  The goal  remains the same; elimination of pain and/or painful loss of motion and restoration of  function.

The role of the first metatarsophalangeal joint is as complex  as any found in the human body.  It is at once, an adaptor and propulsor. The great toe joint is a tri-plane motion enarthrodial joint lined with hyaline articular cartilage. This joint dorsi and plantar flexes in the sagittal plane for propulsion, but additionally adducts and abducts  in the transverse plane , minimally, for stability. Finally, the joint rotates in the frontal plane as the foot pronates and supinates, to accomodate the dual role the foot plays in rigid lever/mobile adaptor function.  The joint is encapsulated in a synovium lined ligament with two sesamoid bones embedded in its plantar structure. These sesamoids belong to the bifurcated tendons of the flexor hallucis brevis and constitute the floor of this capsule. The sesamoids, too, are lined with hyaline articular cartilage.

Through advanced understanding of kinesiology, we now know the role the sesamoids play in force absorption and smooth propulsion. The condition of the articulation between sesamoid and plantar condyle is vital to smooth, efficient motion and some misconceptions must be overcome. In closed kinetic chain motion, the sesamoids do not move. Rather, it is the 1st metatarsal head that  rotates around an axis, over a fixed sesamoid position. The flexor hallucis brevis, originating from the base of the 1st metatarsal is too small to affect any more than a millimeter or two of pull, and therefore serves as a stabilizer of the plantar structures of the 1st MTPJ. Essentially, its proper function is to ensure that the sesamoids remain in a fixed position. In the same model, the proximal phalanx does not dorsiflex on the metatarsal head. The phalanx is fixed to the ground by body weight to extend the moment of propulsion and double limb balance. The metatarsal extends, around its transverse plane axis. Once this is appreciated, it can be understood that the condition of the articular cartilage of the metatarsal and sesamoids is far more important than that of the base of the proximal phalanx. This is stated not to minimize the role of the base of the proximal phalanx, but rather, to emphasize the metatarsal-sesamoidal joint. This author agrees with those investigators who believe that the progenitor of degenerative joint disease in the 1st metatarsophalangeal joint is microfracture and eventual wearing of the articular surfaces of the sesamoids.

The importance of  adequate motion of the 1st metatarsophalangeal joint relative to normal foot function was well presented in Dr. CB Payne's treatise,  The Biomechanics of the Foot in  Diabetes Mellitus(1). He stated that the  sagittal-plane facilitation of motion model(2) suggests that normal foot function is dependent on adequate range of motion at the first metatarsophalangeal joint during dynamic function (which is independent of the range of motion of the joint during clinical examination) so that the windlass mechanism(3) can be established. This allows the foot to adequately resist the stress applied to the foot during the propulsive phase of gait. If this mechanism and other autosupportive mechanisms(2) are not established owing to a functional hallux limitus or inappropriate  direction  of  "weight flow"  through  the  foot or to limited  joint mobility affecting dorsiflexion of the first metatarsophalangeal joint and plantarflexion of the first ray,  a number of compensatory mechanisms are predicted by the model to occur.

Relating  foot function to gait, it is noted that there are a number of requirements for normal locomotion, including antigravity support in the upright posture, balance control and forward stepping movements. The impetus to forward locomotion is achieved by pushing off on the leg in the stance phase while swinging the other leg forward. The "swing" leg is flexed and slightly externally rotated at the hip, flexed at the knee, and dorsiflexed at the foot. The rear stance leg pushes the body forward off its base of support where forward body momentum aided by gravity propels the body(4). Twice during the gait cycle and at very different times, the great toe joint dorsiflexes. During swing phase, the powerful effects of the extensor hallucis longus working in concert with the tibialis anterior and extensor digitorum longus, doriflexes the foot to aid in ground clearance. At heel off, ending stance phase, propulsion is accomplished with dorsiflexion of the great toe joint.

Those who study degenerative joint disease have particular interest in older adults. The incidence of DJD increases dramatically in the fifth, sixth and seventh decades of life.. Changes in the characteristics of gait patterns in older adults are influenced by balance ability, leg muscle strength and altered sensory feedback (4). Two important gait parameters that distinguish between fallers and nonfallers are stride width and floor clearance(4). Balance ability is improved at double limb support by extending the time of that period, stride width is improved, and floor clearance is improved; all a function of stable and smooth 1st metatarsophalangeal joint dorsiflexion. Today, older adults are far more active and health conscious than their predecessors and appear to have less tolerance for compromise of activity.

Complexity of the function of the great toe joint requires special consideration and care when problems affecting this joint are remedied. One must consider the needs of the patient when contemplating treatment.  This consideration should be towards the latest and best, technological medicine has to offer.

The following operative procedure has been developed to include the most recent technique for the Biomet Total Toe System and assumes the condition being treated is 1st metatarsophalangeal degenerative joint disease in end stage arthrosis.This technique is based on the most effectual procedure, following years of shared clinical experience. Where possible, a brief description of the rationale will be given.

The 1st metatarsophalangeal joint is opened via a medial skin incision. Care must be taken to avoid the neurovascular bundle. No underscoring should be necessary. This approach is favored because the surgical technique is medially oriented. It also limits the possibility of  potential wound contracture interfering with plantarflexion during recovery. Lastly, it yields a pleasant cosmetic appearance. The abductor hallucis tendon is continuous with the capsule, medially and is bisected longitudinally, following the fibers. This serves as the capsular incision. This incision should commence approximately mid shaft on the proximal phalanx, traverse the joint and terminate on the distal 1/3rd of the metatarsal shaft. The capsule is underscored  over the metatarsal head, both dorsally and plantarly, freeing the sesamoids from their adhesions to the plantar condyles of the metatarsal head. This dissection technique should be continued to the lateral capsular attachments of the metatarsal.  This so called "degloving" technique is best accomplished utilizing the instruments created for this procedure (refer to McGlamry elevator or Kalish elevator).

The base of the proximal phalanx is carefully underscored both medially and dorsally. With power oscillating saw, resect an appropriate amount of base carefully so as not to penetrate the capsule. Note that you will be installing a 6mm width base component. Be cognizant that the rim of the existing base will be hypertrophic, so measurement should be from the center of the concavity. 6mm should be a minimum measurement. The resected base is then grasped and with a rolling motion, it is "peeled" out of the capsule. One should not be able to visualize the flexor hallucis longus tendon. The sesamoid apparatus located in the plantar capsule is part of the flexor hallucis brevis. This capsule is continuous with the periosteum on the plantar surface of the proximal phalangeal shaft. The base of the proximal phalanx can be resected without causing detachment of the sesamoid, albeit the insertion is thin, requiring extreme care.

Often, in long standing cases of hallux rigidus, the patient has compensated for lack of dorsiflexion of the great toe, by abducting the foot. This may cause an assymetry of the proximal phalanx, so called hallux interphalangeus.  It  can  be corrected  with an oblique phalangeal base resection which is more generous medially. It will lead to improved function, alignment of the component stem in the long axis of the proximal phalanx and cosmesis.

The metatarsal head is now prepared for implantation. The medial eminence is resected via an easy technique  using  a large osteotome, but removing only a small amount of eminence and insuring that it is in the proper planes. This resection must be equidistant from distal to proximal and dorsal to plantar. The newly created surface is the staging area for all transverse metatarsal cutting and therefore is critical.

A helpful alternative procedure is described utilizing a dedicated guide. Retract the metatarsal head medially and locate the center of the frontal view. Drive a .045 Kwire into the head aligning the long axis of the metatarsal. Cut the wire short and place on it, the star shaped captured cutting jig with the capture apparatus postured medially. Align the axis of the jig in the sagittal and transverse planes. Slide the jig on the wire to the appropriate implant size (xs to l) and set the jig's teeth into the articular surface. Insert an appropriate length oscillating saw blade into the capture slot and begin the medial eminence resection. The resection must be in the sagittal plane from dorsoplantar and proximodistal because the guides that prepare the metatarsal for implantation are set on this resected surface. This guide,used properly, will insure an accurate surface. Remove the Kwire.

The sagittal plane cutting guide is now positioned on the metatarsal head. The guide should be aligned in the long axis of the metatarsal shaft, no less than 4mm proximal to the subchondral line. If  "slack" in the joint is desired or if the 1st metatarsal is long, more resection is recommended,however be aware of the relationship in length to the second metatarsal and the consequences of a short 1st metatarsal. The guide can also be adjusted to accommodate for elevatus of the metatarsal. The guide may be positioned plantargrade in the long axis ( not angulated, but rather translocated plantargrade) , several mm's. This serves to relocate the joint plantarly. Once position is established, fix the guide in place with three .045 Kwires. Cut the wires shorter and remove the guide. Install corresponding capture jig on the wires and tap the jig up to the bone. With  an appropriate oscillating saw blade, make the four transverse cuts. Allow the saw to do the work. Bearing down will cause the blade to "flair" the cuts laterally. Remove the jig, the Kwires and the capital fragments.

The frontal plane drill guide has the same configuration and dimensions as the internal surfaces of the metatarsal component to be installed. This is an excellent opportunity to check the fit for exactness. If minor adjustments need to be made, make them now. This guide is then positioned medial to lateral and utilizing a 1.5 mm wire passer an introductory hole is established (avoid using a burr, as it can grind the edges of the guide thus introducing metal fragments into the wound.) This hole need not be deep, as the starter broach will accomplish this goal. Use the starter broach to increase the dimensions of the hole. Pay careful attention to the alignment of the broach in all three planes. This is the number two cause of implant misalignment and is easily avoided. A finishing metatarsal broach is employed to create an intramedullary hole 10% smaller than the stem of the designated sized metatarsal implant. Be certain to choose the right size broach. Tap the broach until the stop collar rests on cancellous bone. On occasion, the cancellous bone will be extremely dense and difficult to broach. It is then recommended to gradually develop the hole with alternate use of power wire pass and broach. In these special circumstances, the beveling of the canal's opening edge is advised to accommodate the slight radius at the junction of stem and interface on the inside of the component.

The phalangeal intramedullary hole is now developed. The phalangeal drill guide is positioned over the resected phalangeal shaft. Once again, an opportunity to check the fit to be certain the phalangeal component will cover all cortices but not be too large for the joint. Center the guide and drill a hole into the cancellous bone with the power 1.5 mm wire passer. Remove the guide and cover the hole on center with the starter broach. Into this slot, introduce the square tapered phalangeal broach of appropriate size and tap the broach to the stop collar. Again, if cancellous bone is dense, develop slowly alternating power drilling and broach.

Attention is now given to the sesamoids. It should be anticipated that they are irregular and hypertrophic. Be prepared to remove the lateral sesamoid (The rationale for lateral (fibular) sesamoid removal is, 1) the lateral sesamoid is difficult to plane and may be extremely hypertrophic which can cause plantar instability when the sesamoid is returned to its anatomically correct position. 2.) In the presence of hallux abducto valgus and/or hallux limitus, a plantar capsular lengthening would be welcomed. By removing the sesamoid , a round incision is readily converted to an ellipse with medial angulation of the hallux, thus adding length to the lateral plantar capsule.) The medial sesamoid should be modified by isolating its periphery with a superficial circumscribing incision . Skin hooks will retract the plantar capsule while a small bone rongeur is used to skive and shape the irregular edges. The remaining bulk of the sesamoid can be reduced by removing the articular surface. Be careful not to overzealously reduce the sesamoid, as it will be weightbearing and can fracture. It should be approximately 5mm thick.

Irrigate the surgical site and install the metatarsal component, driving it to seat with the  polyethylene faced driver and mallet. Use a mallet with some "heft",and progress slowly. The phalangeal component is installed by plantarflexing the great toe. The implant is inserted , the joint realigned, and retrograde pressure drives the implant to seat.

When the lateral capsule was released early in the technique, it allowed for the all important ligamentous balancing. No capsulotomies should be necessary. Repairing the medial capsular incision with tension adjustment, should bring the great toe into balance. Removal of the lateral sesamoid should give a lateral release in tension. This is providing the weight bearing intermetatarsal angle is correct. If it is not, ancillary osteotomy should be performed. Osteotomies should be anticipated prior to surgery and accomplished after the implantation    technique as the osteotomy may not be able to withstand the tapping necessary in this technique.

• Establish range of motion
• On table postoperative radiograph, AP (with pushup pressure) and lateral
  Check implant alignment and seat
  Check joint alignment
• Adequate dressings, but light enough for early passive ROM
• Antibiosis
• BK cast for
  Ancillary osteotomy
  Questionable compliance
  

• Review wound
• Remove sutures
• Assess discomfort
• Assess active and passive ROM
• Return cast, as indicated
• Assess dependency on foot and advise
  

• Radiographs AP and lateral (weightbearing)
• Assess active and passive ROM
  Passive motion important to extension function
  Active motion, important plantarflexion as resistance to ground
• Assess discomfort relative to weightbearing
• Assess edema, hard and soft
• Return to reasonable footwear, consider progress shoe style
  

• Assess ROM, active and passive
• Assess edema
• Assess shoegear
• Assess activity
  

• Radiographs AP and lateral (weightbearing)
• Assess ROM
• Assess Activity
  
  

References

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