A Closer Look At The Primus First MPJ Implant

First metatarsophalangeal (MPJ) implants have been in widespread use since the 1970s. While there was early use by Seeburger, Joplin, Swanson, Townley and others, it was not until Dow Corning introduced the Swanson design, Silastic hemi-implant, that these implants became commonplace.
Throughout their history, the use of the various first MPJ implants has been in a constant state of flux. While there have been many surgeons who continued using the newer design implants, there are also many surgeons who abandoned implant use in favor of alternative procedures.

A Guide To Indications And Contraindications
Since the earliest days, there have been no changes in the indications for first MPJ implants. Patients with hallux limitus/rigidus have been the primary beneficiaries of implant arthroplasty. Patients suffering from hallux valgus in association with degenerative arthritis, painful rheumatoid arthritis and unstable and/or painful joints from previous surgery have also been considered candidates. The geriatric bunion is now another condition that responds well to implant arthroplasty.

Severe bone demineralization or inadequate bone stock prohibit use of these implants. One must also give consideration to the neurovascular status and adequacy of the musculotendinous structures. Do not use implants in patients with active sepsis. There may be patients who are psychologically unsuitable for this type of surgery.

How Implant Design Has Evolved
The evolution of first MPJ implant designs has been a somewhat gradual process. There are three generations of these implants.22
First generation. These implants arose out of a need for another alternative for the various degenerative processes of the first metatarsophalangeal joint. The Keller procedure with and without soft tissue interposition can relieve pain but often results in an unstable joint, loss of hallux toe purchase and lateral migration of weightbearing. Arthrodesis has been successful in eliminating pain but sacrifices joint mobility, leads to degenerative changes in the adjacent joints and may limit shoe gear.

First generation implants were generally either modified hand implants or simple joint spacers. There was no specific instrumentation. Surgical technique varied widely and results were inconsistent. Often the same joint instability, loss of hallux toe purchase and lateral migration of weightbearing were still present. Over time, joint and bone changes such as detritic synovitis and fibrous hyperplasia were present. Vanore, O’Keefe and Pikscher published a classification system for the observed complications.23
Second generation. Several second-generation designs were introduced during the 1980s and early 1990s. Many of the implants were double stemmed with hinges to improve joint stability and provide range of motion. They were designed to be much more joint specific and considered the unique anatomy of the first metatarsophalangeal joint. Dow Corning, Sutter Biomedical and Sgarlato Labs advocated using silicone elastomers for their hinge designs.
Due to the silicone breast implant litigation, other companies chose to discard silicone and developed two component designs of metal and ultra-high molecular weight polyethylene. The two component systems were much more technically difficult to perform and the results were mixed at best, resulting in only limited use.
Third generation. Decades of experience coupled with technological advances led to the third-generation implants. These designs incorporate advanced materials and are computer modeled to consider the static and dynamic joint specific anatomy. They are highly durable and have simple, accurate instrumentation.
In 1997, Futura Biomedical introduced UltraSIL silicone elastomer for its flexible implants. The material exhibits advanced physical properties over previous materials in the critical areas of tensile strength, tear resistance and abrasion resistance.

Using Computer Modeling To Facilitate Improved Treatment
Using computer modeling, Futura Biomedical also began designing the Primus, the first third-generation, double-stemmed implant for the first metatarsophalangeal joint. Computer modeling provides the ability to compare various design options before proceeding with the optimum design.

Of particular interest, the computer models demonstrated that all of the second-generation double-stemmed implants had a design flaw. They had been designed like hand implants with the faces and stems coming off the hinge opposite each other. Everyone had anticipated that the metatarsal and phalanx both would sit on the ground in weightbearing. No one considered that the first metatarsal sits on the sesamoid complex and is actually somewhat dorsal to the base of the proximal phalanx.
One of the design features of the Primus implant is an axially offset hinge with the metatarsal side of the implant somewhat dorsal to the phalanx side of the implant. This design is anatomic and is a patented feature. The hinge allows for 95 degrees range of motion and has been engineered and tested respecting the anatomic axis of motion proximal to the center hinge. There is a patented, unique plantar strength rib. Precise instrumentation is available to make accurate cuts and preserve the anatomic insertion of the flexor hallucis brevis tendon.

Case Study: Treating A Bunion In A Geriatric Patient
A 68-year-old female patient presents with severe, longstanding, painful bunion deformity of the left foot. Weightbearing X-rays reveal osseous hypertrophy of the first metatarsal head with an intermetatarsal angle of 19 degrees and hallux abductus of 39 degrees. After discussing various surgical options with the patient, it was elected to perform implant arthroplasty. The surgeon felt that this procedure would give greater stability and improve the function over a Keller bunionectomy. At two years post-op, the patient has maintained correction and has a pain-free, functional joint.
With previous generation implants, this patient would not have been a candidate for implant arthroplasty. Shorter stems and a more anatomic design now allow this option for geriatric bunions. The important angle to consider is not the intermetatarsal angle but rather an unnamed angle that is measured at the intersection of the long axis of the implant and the second metatarsal bisection. If this angle is close to 0 degrees, then there is minimal stress on the implant. It is also necessary to release all soft tissue contractures.
In most cases, this will include release of the lateral head of the flexor hallucis brevis tendon from its insertion into the base of the proximal phalanx. It is helpful to free the medial joint capsule from the subcutaneous tissues. This should begin at the dorsal incision and continue plantarly to the inferior aspect of the tibial sesamoid. This release will help mobilize both the joint capsule and the flexor complex, and in combination with excision of redundant dorsal medial capsule, allows for reduction of the angular deformities.