Using Injectable Grafts To Facilitate Implant Success
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.