One of the most commonly performed procedures in the foot is an implant arthroplasty of the first metatarsophalangeal (MPJ) joint. However, we have occasionally noted a hollow medullary canal after resecting the base of the proximal phalanx. This can cause problems with incorporation of the implant. Yet there is an intraoperative option one can use to address this problem.
Adequate bone stock of the proximal phalanx is crucial for proper seating of the hemi-implant. In some cases, we have found the cancellous bone to be cystic with liquefied fat. However, there has been no evidence of cortical disruption. Keep in mind that one cannot see these intra-operative findings initially on radiographic examination.
When performing an arthroplasty of the first MPJ, it is important to remove enough bone to not only provide space for the implant but also to decompress the joint. Adequate resection of the proximal phalanx base is essential for relaxing the plantar structures and providing enough dorsiflexion during the propulsive phase of gait. It is important to exercise caution and avoid sacrificing the plantar structures that insert in the proximal and distal phalanges.
What You Should Know About The Medullary Canal
One would perform the resection by placing the sagittal saw blade parallel to the first MPJ and perpendicular to the shaft of the proximal phalanx. Occasionally, after resecting the proximal phalanx base, we have noted the cancellous bone is soft in texture and yellow in color. With manipulation, this bone liquefies and oozes from the canal, resulting in a hollow proximal phalanx.
The basic properties of bone are needed to provide adequate implantation of any device. The medullary canal must be strong enough to hold the stem of the implant and prevent displacement, improper positioning or movement during the gait cycle. Furthermore, the cortical bone must be strong enough and the implant must be properly sized in order to prevent bulldozing of the implant. This phenomenon occurs due to compressive forces against the implant, which is smaller than the cortical margins of the phalanx.
When poor medullary support exists, there are a few options you can utilize. The most likely scenario is removing the base of the proximal phalanx without the use of an implant (via a Keller bunionectomy). However, over the past few months, we have used Minimally Invasive Injectable Graft (MIIG) to maintain the position of the implant in the proximal phalanx.
How To Use The MIIG
MIIG (Wright Medical) is an injectable graft of surgical grade calcium sulfate that has been formulated to replace defects in metaphyseal bone and provide temporary structure for hardware fixation in fractures. MIIG uses OsteoSet, the biosynthetic calcium sulfate formulation for bone regeneration. This osteoconductive material completely resorbs at a rate consistent with new bone formation.
By placing MIIG into the canal, an appropriate consistency for implantation occurs within two to five minutes and one can insert the stem of the implant. The hardened MIIG acts as a support for the implant and provides necessary stability for function.
Prior to mixing the graft, it is important to prepare the phalanx and select the proper size of the implant. Afterward, one can prepare the MIIG graft. After roughly 60 seconds of mixing, you would dock the syringe onto the needle and then inject the MIIG into the defect. Allow two minutes for some hardening of the graft, but be aware that it must be soft enough to allow incorporation of the hemi-implant.
We have found approximately 4 cc is needed in the proximal phalanx. One should leave approximately 2 mm of space at the base of the resected area in order to prevent displacement of the MIIG liquid into the surrounding tissues as you insert the implant. After inserting the hemi-implant, wipe away any excess expunged from the site and allow the graft to dry and harden. Make sure there is good bony contact between the implant and the cortices of the proximal phalanx. Remember that the mechanical properties of the graft do not change when you insert the hardware.
A Closer Look At The Results
While radiographic findings are inconclusive in determining if bony regrowth has occurred (due to MIIG being radiopaque), we have observed clinically that the first MPJ functions as it would with the use of any other titanium hemi-implant. There is adequate dorsiflexory motion of the great toe and patients are able to stand on their toes.
Furthermore, radiographs have shown that the implant remains in adequate position three months after the procedure. There has not been any bulldozing or loss of stability of the implant, and the position of the implant in the proximal phalanx base has not changed.
Our patients have not experienced any postoperative infections, metal reactivity or detritic synovitis. Whether the bone regenerates with the assistance of the calcium sulfate or the MIIG is not resorbed and replaced by bone, the implant still has a stable, well-controlled seated position for adequate range of motion during propulsion.
What One Case Study Revealed
A 47-year-old woman came in to the office, complaining of severe pain in the first MPJ. The patient had multiple cheilectomies with temporary relief lasting only six months to one year. She notes recent difficulty with ambulation and it had altered her life accordingly.
Upon physical examination, we noted the patient had findings consistent with hallux rigidus along with a significant dorsal exostosis. The range of motion of her first MPJ was limited to approximately 2 degrees of dorsiflexion. Radiographic evaluation revealed a markedly decreased joint space, flattening of the first metatarsal head, subchondral sclerosis and osteophyte formation.
We directed our surgical attention to the dorsal aspect of the foot, making a curvilinear incision over the first MPJ. We proceeded to remodel the first metatarsal head and removed the base of the proximal phalanx with a micro-sagittal saw.
Upon inspection, we saw the medullary bone was absent, leaving a shallow cortical rim with poor medullary support for the hemi-implant. Accordingly, we prepared MIIG and introduced it into the proximal phalanx base. We allowed it to harden for approximately three minutes to the appropriate consistency. At this point, we inserted the properly sized hemi-implant into the base of the proximal phalanx. We noted adequate seating and range of motion intraoperatively.
Arthroplasty of the first MPJ with implant placement is an excellent procedure for patients with degenerative arthritis. This procedure provides marked improvement in range of motion, significant reduction in painful symptoms and maintains the length and function of the great toe.
However, some patients have less than adequate bone stock for implantation. With the technological development of MIIG, we are now able to provide an environment for proper seating of the implant. When you’re treating patients that would otherwise require a Keller arthroplasty or fusion, the option for restoring motion still exists. MIIG is an excellent alternative approach to implant seating in light of poor proximal phalangeal support.
Dr. Harold Schoenhaus is the Chief of Foot and Ankle Surgery at the Graduate Hospital of Philadelphia. He is a Fellow of the American College of Foot and Ankle Surgeons, and is board-certified by the American Board of Podiatric Surgery and the American Board of Podiatric Orthopaedics.
Dr. Jodi Schoenhaus is a second-year resident at the Graduate Hospital in Philadelphia. Dr. Dawn Pfeiffer is a third-year Chief resident at the aforementioned institution.
Dr. Burks is a Fellow of the American College of Foot and Ankle Surgeons, and is board-certified in foot surgery. Dr. Burks practices in Little Rock, Ark.