Revisiting The Silastic Implant For The First MPJ

Hallux rigidus is a progressive, degenerative process that results in decreased or absent motion of the first metatarsophalangeal joint (MPJ), with accompanying pain and crepitus.¹ The frequency of this condition has led to an abundant selection of conservative and surgical treatment options. Proposed procedures for surgical treatment of hallux rigidus include cheilectomy, resection arthroplasty, arthrodesis and prosthetic joint replacement.²

Swanson first introduced Silicone arthroplasty in 1968 as an interpositional single-stem, hemi-implant for the great toe.³ These implants were modeled after the silicone implants used in the hands and were designed to improve the Keller arthroplasty.^4,5 Unfortunately, the implants were not able to tolerate weightbearing forces, and the failure rate was high.⁴

Over the next decade, this concept further developed into the double-stem, hinged silicone implants we are familiar with today. The new designed prevented the silicone material from articulating with the joint.⁶ Manufacturers selected silicone as the biomaterial of choice in these implants due to its inert nature and softer composition.³ The belief was that these implants would ultimately be better tolerated and less likely to damage adjacent bone.³ A benefit of using the silastic joint over performing a Keller is that the implant allows the hallux to maintain its length.^3,7

A Closer Look At The Indications For Silastic Implants

One may consider a silastic implant when there is significant arthritis with narrowing and pain of the first MPJ.⁸ When considering a Keller as a surgical treatment option, a double-hinged silicone implant is a viable alternative.⁸ The goals of surgery are to maintain motion of the joint, maintain length, and relieve pain. The ideal candidate is a middle aged or elderly patient who is lightly active but wishes to maintain motion of the first MPJ.³ A specific example of a patient who received this joint at our institution is an elderly woman who kneels down while gardening.

A double-stemmed implant may correct a flexible, mild hallux valgus deformity. However, one should not use these silastic implants to correct a large first intermetatarsal angle of greater than 16 to 19 degrees.³ Rather, one should perform concomitant soft tissue and/or osseous procedures to address this before putting the implant in place. The implant does offer some transverse plane stability, but is often unreliable with severe hallux valgus deformities.³

We can attribute many complications encountered with silastic joints to poor patient selection or surgical technique. When discussing the indications for use, it is important to note that the silicone implant is intended to act as an interpositional arthroplasty, rather than as a total joint replacement procedure.³ We must counsel patients prior to surgery on their expectations and outcomes postoperatively. Generally, patients must be willing to permanently give up certain exercises, such as running, tennis or other activities, that place significant stress on the first MPJ.⁷ Thus, avoid using these implants in highly active patients.

A Guide To Surgical Technique

Place the surgical incision over the dorsomedial aspect of the first metatarsophalangeal joint in a linear fashion. Carry anatomic dissection down through skin and subcutaneous tissue to allow visualization of the capsular structures. Make a linear incision, and preserve and reflect the capsular tissue. Then obtain complete exposure of the base of the proximal phalanx and the head of the first metatarsal. One may resect any exophytic bone growth using a rongeur or bur.

At this time, resect the base of the proximal phalanx using a sagittal saw. Perform this by removing approximately 1/3 of the proximal aspect of the phalanx. With most implant systems, take care to perform the osteotomy perpendicular to the long axis of the hallux or parallel to the eponychial nail fold.³

Then direct attention to the first metatarsal head, where one performs resection of the metatarsal articular surface, again using a sagittal saw. The cut should be perpendicular to the weightbearing surface, taking care not to perform this at an angle. Any angulation of this can result in failure of the joint.

Intramedullary canal preparation begins by using a 4.0-mm football or oval bur in the metatarsal or the phalanx. One should be cautious not to over-drill the canal, as this will result in movement of the implant and ultimately, failure of the joint. Place the canals slightly dorsal within the central aspect of the metatarsal and phalanx. This will help to prevent these two bones from colliding with dorsiflexion. Follow this with careful enlargement of the canal to the appropriate size. Depending on the implant system, one may utilize reamers or rasps for enlargement. These are typically tapered in a manner that resembles the stem of the implant.

After preparing the canals on the metatarsal and phalanx, place a trial implant. After determining a size, insert the final implant. Silicone implants come with titanium grommets, which in theory prevent breakdown of the implant.³ At our institution, we forego use of the titanium grommets. It has been our experience that patients commonly undergo metallosis from the grommets and require removal of the implant.

Pointers On Results And Complications

A discussion of the silicone arthroplasty is not complete without reviewing its results and potential complications.

Perhaps the most worrisome complication is that of silicosis. There have been reports of silicone lymphadenopathy and systemic silicosis associated with first MPJ hemiarthroplasty. A retrospective review by Rahman and Fagg revealed signs of silicone granulomatous disease in 56 out of 78 feet at 4.5 years post-op following a silicone hemiarthroplasty.⁹ They recommended complete abandonment of this procedure for hallux valgus or hallux limitus. This study, and others like it, led to the development of a double-stem implant.

Authors have cited a number of potential complications, such as bone overgrowth, loosening of the implant, continued pain, and the loss of toe purchase. Frequently cited is the study by Granberry and colleagues, a retrospective review of 90 total joint replacements with a silicone hinged prosthesis.² In their review, the authors evaluated patients at an average of three years postoperative. Of the patients included in the study, 25 feet had failed resection arthroplasty, 14 feet had severe hallux rigidus, 21 feet had rheumatoid arthritis, 12 feet had failed bunionectomy, 16 feet had failed hemiarthroplasty, one foot had a chronic infection, and one foot had a shotgun wound. The most common complication observed in this study was that of radiographic failure of the implant due to fracture without dislocation. This occurred in 29 percent (21 feet). Despite implant failure, the authors did note that the majority of patients remained pain free and were satisfied with their surgical outcome. Patients rated their results as good or excellent in 74 percent (67 patients) and pain as absent or slight in 64 percent (41 patients).

There are some shortcomings to this study. The authors did not appear to provide any activity limitations to their patients postoperatively. This may be the reason they cited a significant number of implant fractures. It is also important to point out that in this study, 25 of the 90 feet reviewed had received a resection arthroplasty and 16 feet had a failed hemiarthroplasty that failed prior to receiving an implant arthroplasty.² I would propose the question that if patients fail resection arthroplasty, are they at a greater risk to fail an implant arthroplasty?

Another study by Morgan and colleagues evaluated the long-term results of the double-stem silicone implant in the treatment of hallux rigidus.⁵ They reviewed 108 feet in 83 patients with an average follow-up of 8.5 years. In 82 feet (78 percent), patients reported no pain postoperatively, and only five feet (4.8 percent) reported occasional pain. The greatest complication encountered in this study was transfer metatarsalgia, reported in 12 feet (11 percent). The authors did not report any cases of silicosis. They concluded that silastic implant arthroplasty is a viable procedure for hallux rigidus with high patient satisfaction.

As these implants continue to evolve and improve, more research will be necessary to fully evaluate the complications, and the short- and long-term outcomes.

References

1. Kim PJ, Hatch D, DiDomenico LA, et al. A multicenter retrospective review of outcomes for arthrodesis, hemi-metallic joint implant, and resectional arthroplasty in the surgical treatment of end-stage hallux rigidus. J Foot Ankle Surg. 2012; 51(1):50-56.
2. Granberry WM, Noble PC, Bishop JO, Tullos HS. Use of a hinged silicone prosthesis for replacement arthroplasty of the first metatarsophalangeal joint. J Bone Joint Surg Am. 1991; 73(10):1453-1459.
3. Vanore JV. Implant arthroplasty at a crossroads. In Ruch J and Vickers D (ed.), Reconstructive Surgery of the Foot and Leg - Update '92. Podiatry Institute, Inc., Tucker, GA, 1992, chapter 37. 
4. Johnson MD, Brage ME. Total toe replacement in the united states: what is known and what is on the horizon. Foot Ankle Clin. 2016; 21(2):249-266.
5. Morgan S, Ng A, Clough T. The long-term outcome of silastic implant arthroplasty of the first metatarsophalangeal joint: a retrospective analysis of one hundred and eight feet. Int Orthoped. 2012; 36(9):1865-1869.
6. Lawrence BR, Thuen E. A retrospective review of the primus first MTP joint double-stemmed silicone implant. Foot Ankle Spec. 2013; 6(2):94–100.
7. Cracchiolo A 3^rd, Weltmer JB Jr., Lian G, et al. Arthroplasty of the first metatarsophalangeal joint with a double-stem silicone implant. Results in patients who have degenerative joint disease failure of previous operations, or rheumatoid arthritis. J Bone Joint Surg Am. 1992; 74(4):552-563.
8. Bouchard JL, Phillips AJ. Implants: total vs. hemi. In Ruch J, Vickers D (ed.), Reconstructive Surgery of the Foot and Leg. Podiatry Institute, Inc., Tucker, Ga, 1989, chapter 35, pp. 313-320.
9. Rahman H, Fagg PS. Silicone granulomatous reactions after first metatarsophalangeal hemiarthroplasty. Bone Joint J. 1993; 75(4):637-639.
10. Roukis TS. Nonunion after arthrodesis of the first metatarsal-phalangeal joint: a systematic review. J Foot Ankle Surg. 2011; 50(6):710-713.