Current status of silicone joint implants for first MTPJ replacement

It has been about six decades since surgeons started using silicone implants for the first metatarsophalangeal joint, with patients suffering from hallux rigidus being the main beneficiaries. Mohammad Salim, Elizabeth Alderton and Rajiv Limaye ask, is its use still a viable option in the 21st century?

History

As surgeons started recognising the importance of the first metatarsophalangeal joint (MTPJ) in foot function and maintaining normal gait, interest emerged in developing a functional first MTPJ prosthesis. In 1952, Swanson designed a metal hemispherical cap with a tapered stem as a first metatarsal head replacement [1]. Swanson later believed the implant failed because of the rigidity of the material [2], prompting him to design a single-stemmed intramedullary prosthesis made of only silicone in collaboration with Dow Corning in 1967 [3].

The implant took cues from the existing hand implants and was primarily designed to act as a spacer to augment the Keller’s procedure [4]. Silicone was selected as the material of choice because of its inert nature, softer composition and well-tolerated physiologic properties [5].

The hemi-implant was not free of complications however, the most worrisome being silicosis which included dentritic synovitis, silicone lymphadenopathy and silicone synovitis. This was reported by Rehman and Fagg in their study [6] in which they recommended abandoning the procedure entirely. This led to the development of the double-stemmed hinged implants, which provided better joint stability and range of motion [7]. Subsequently, titanium grommets were introduced to protect the hinged implants from bone damage [8]. This double-stemmed hinged silicone implant is still the most commonly used and accepted prosthesis with an increase in its utilisation recently [5,7].

Indications

The general patient profile for the use of the silicone MTPJ prosthesis is a middle-aged to older individual with low to moderate activity levels [7]. Hallux rigidus remains the most common indication for its use with the aim to relieve pain, maintain joint motion and maintain length [9]. Other indications include rheumatoid arthritis and hallux valgus associated with arthritis. However, it is not recommended to use the implant where the intermetatarsal angle exceeds 16-19 degrees [5]. It is also worth mentioning that smaller silicone implants have been used for treating Frieberg’s disease and second and third MTPJ arthritis as well.

Silicone implants are an interpositional arthroplasty and act as a spacer. They are not a joint replacement procedure as such [5,9]. The implant gets encapsulated and forms a capsuloligamentous system to provide stability [10]. As time goes on the joint becomes flexible and functions as a new joint.

Surgical technique

After positioning the patient supine with a tourniquet and sand bag on the ipsilateral side, a dorsomeal approach to the first MTPJ is utilised. Skin and subcutaneous dissection is carried out in line with incision up to the joint capsule.

The capsule is incised in a linear fashion and reflected to provide adequate exposure of the metatarsal head and base of the proximal phalynx.

Osteophytic over growth is excised and adequate bone cuts are taken. The phalangeal cut is performed parallel to the eponychial fold, while the metatarsal cut is performed perpendicular to its weightbearing surface. Resection must be kept to a minimum to avoid shortening and transverse metatarsalgia [5].

Initial entry into the intramedullary canal is made using a 2.5mm drill. Then the canal is prepared using an introducer and increamental broaches.

After the preparation of the canal, a trial prosthesis is inserted to judge the required size and adequacy of the resection. This is followed by implantation of the actual prosthesis. The stems of the prosthesis can be cut to allow better seating of the implant.

Titanium grommets are available which, in theory, protect the implant from bone damage. But using them is entirely at the surgeon’s discretion.

If required, dorsal cheilectomy may be performed following implantation to help achieve better range of motion.

Our experience

30 patients with a mean age of 60.5 years (45–76 years) were reviewed over a span of 18 months. All of these patients received Swanson’s toe replacement (Wright medical technology Inc). A total of 33 prosthesis were implanted (three bilateral and 27 unilateral). Co-morbidities included non-insulin dependent diabetes (three patients), hypertension (five patients) and fibromyalgia (two patients). Most of the patients in our study group were within average BMI range (20–25). All patients completed VAS and (American Orthopaedic & Ankle Society Score) AOFAS pre-operatively and post-operatively.

We have already described our surgical technique earlier and all of our patients were treated as a day case surgery. They all received ankle block and GA. Post operatively they all were mobilised heel weight bearing up to two weeks and thereafter full weight bearing. They all received two weeks of DVT prophylaxis until they were fully mobile. Sutures were removed at two weeks post-operatively and physiotherapy commenced. Unprotected weight bearing was allowed after two weeks and patients started wearing normal shoes as tolerated after six weeks. We did not use the titanium grommets in any of our cases.

All patients had routine follow-up at six weeks, three months, six months and 12 months with clinical assessment and weight bearing radiographic evaluation in each review. Radiographs were deemed satisfactory if they demonstrated full deformity correction, good implant position and no intra-operative fractures. Repeat assessment of the VAS score and AOFAS score was done on the final review before discharge.

The mean VAS score was 7.82 with a mean AOFAS score of 22 pre-operatively. The range of motion arc at the MTPJ pre-operatively had been between 10–30 degrees of plantar and dorsi flexion. The VAS score post-operatively dropped to two in all patients within three months and their range of movement arc was within 20-90 degrees of plantar and dorsi flexion post operatively, achieving a fantastic outcome. Post-operative AOFAS scores improved to an average of 77 (61-93). 90 per cent (30 implants out of 33) post-operative check radiographs were deemed satisfactory. All patients were able to wear regular footwear following the procedure. Only one patient needed removal of the prosthesis due to concerns of deep infection. However on deep tissue cultures, no growth was seen and the patient is awaiting revision into a second stage MTPJ fusion.

Complications of the silicone first MTPJ implant

Silicosis, bony over growth, osteolysis, implant fracture and transverse metatarsalgia have been reported with the use of the silicone first MTPJ implant. But the only problems we encountered in our study were bony changes (osteolysis/ bony over growth) in three patients and suspicion of deep infection in one.

As mentioned earlier, silicosis was mainly associated with the use of hemi-implants [6,11]. A retrospective study of 108 feet published by Morgan et al reported no incidence of silicosis with the double-stemmed hinged implants [10].

Bony changes such as osteolysis and bone proliferation at the margins of the resected bone are observed commonly [5]. But these radiographic changes do not correlate with the patients’ subjective satisfaction and functional outcomes [10]. We observed a variable amount of lysis in the follow-up radiographs of three patients (10 per cent), this deterioration however did not correlate with reduction in patient satisfaction.

Granberry et al reported a 29 per cent incidence of radiographic implant failure without dislocation in their study. Despite this, the majority of their patients remained pain-free and satisfied with the surgical outcome [12]. This may be explained by the fact that the implant provides stability via encapsulation, with the implant acting as a template for deposition and alignment of encapsulating collagen tissue [10,13].

Morgan et al reported an incidence of 11 per cent of transverse metatarsalgia following first MTPJ replacement with the silicone hinged prosthesis [10]. But none of our patients showed any evidence of this. Therefore, we reiterate the importance of minimal bone resection in a bid to possibly avoid this complication.

Discussion

Arthrodesis is still the preferred treatment for end-stage hallux rigidus in young and active patients. Silastic first metatarsophalangeal joint replacement is an accepted treatment for hallux rigidus, which not only provides long-term pain relief but also satisfactory range of movement.

In our series, most patients reported good pain relief at six months after the operation, and improved function in activities of daily living and work. All patients reported no difficulty in walking or jogging, although there was some limitation of recreational activity.

Hetherington et al in 1993 described similar good functional results to Granberry et al [1,12]. Moreover, they also reported some cases of implant migration and fracture, which we did not observe in our series.

Lemon et al in 1997 described a study of 55 cases of silastic implants, and they stressed the importance of realigning the joint via appropriate osteotomies and proper soft tissue balancing for better outcomes [15]. They reported a 91 per cent success rate of the surgery in 55 patients, where 82 per cent would be willing to undergo the procedure again and 85 per cent were willing to recommend the procedure to family and friends.

Conclusion

We conclude that silastic flexible hinge (two-stem) first metatarsophalangeal joint replacement is an effective treatment for end–stage hallux rigidus, which not only provides long–term pain relief but also a satisfactory range of movement. Therefore, it should still be considered as an option in selected patients where other total joint replacement or arthrodesis is not feasible. We recommend a long-term large multicentre study to evaluate this further.

References

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Authors: Mr Mohammad Salim – Foot and Ankle Fellow

Miss Elizabeth Alderton – Surgical Care Practitioner

Mr Rajiv Limaye – Consultant Orthopaedic Surgeon

University Hospital of North Tees and Hartlepool, UK

Staff Writer