Microvascular surgery: The evolving role of vascularized bone grafts for challenging scaphoid nonunions
Scaphoid fractures are the most common wrist fracture involving the carpal bones. This fracture is notoriously difficult to treat and at risk of nonunion due to multiple factors, including significant bone loss, carpal collapse, avascularity, arthritic changes, systemic factors, adequacy of fixation and failed previous surgery. Nonunions have been reported in 5 to 15 percent of all acute fractures of the scaphoid.
Scaphoid nonunions with both humpback deformity and avascular necrosis (AVN) are particularly challenging. Both problems need to be corrected for patients to experience substantially reduced pain, improved strength and recovered range of motion. AVN is estimated to develop in 3 percent of all scaphoid fractures, most often in the proximal pole of the scaphoid due to its retrograde intraosseous blood supply.
"Conventional nonvascularized bone grafts work well, with 80 to 84 percent union rates reported, to stimulate healing and correct scaphoid length, but they are less effective after failed previous surgery and AVN of the proximal pole," explains Allen T. Bishop, M.D., Mayo Clinic orthopedist at the Rochester, Minnesota, campus specializing in microvascular surgery. In these settings, union rates with nonvascularized grafts range from 40 to 67 percent.
"Vascularized bone grafts (VBGs) have a role to play in some scaphoid nonunions, but are not a panacea for all problems," Dr. Bishop adds. "Clinical and experimental studies continue to clarify selection of optimal treatment in a variety of clinical circumstances."
Vascularized bone may be harvested on a vascular pedicle from adjacent bones (dorsal or palmar distal radius, second metacarpal) or as a free microvascular transfer from the medial femoral condyle or iliac crest. Selection of appropriate grafts has been guided by a number of studies, including several from Mayo Clinic.
Distal radius pedicle grafts
While Mayo has long been a leader in use of vascularized bone grafts to reconstruct large segmental bone loss in many locations, including the spine and long bones of the upper and lower extremities, their use for hand and wrist pathology has recently had proven value. Dr. Bishop and his team described a pedicle graft from the distal radius that has proved effective in proximal pole scaphoid nonunions with avascular necrosis in short- and long-term studies.
"Importantly, however, we have found this method, based on the 1,2 intercompartmental supraretinacular artery, to be less than ideal in two clinical circumstances: when there is carpal collapse with a scaphoid humpback deformity combined with avascular necrosis, and when the proximal pole is fragmented," he explains.
To expand the options and improve outcomes of difficult scaphoid nonunion procedures, Mayo Clinic orthopedic research focuses on innovative uses of VBGs that address both avascularity and carpal collapse. Chief among them are refinements to free vascularized medial femoral condyle grafts and use of a new technique using osteocartilaginous bone flap reconstruction of proximal pole scaphoid nonunions.
Free vascularized medial femoral condyle
A free vascularized medial femoral condyle (MFC) graft provides not only structural support but also the robust, consistent blood supply needed to help restore both scaphoid geometry and excellent function.
"Free vascularized bone graft taken from the medial femoral condyle demonstrates significant promise in addressing complex scaphoid nonunions," Dr. Bishop says. Data show that in the setting of AVN and carpal collapse, the free MFC VBG resulted in a 100 percent rate of union (n = 12) as compared with 40 percent (four cases out of 10) with use of the 1,2 intercompartmental supraretinacular artery (1,2-ICSRA) graft.
Adds his Mayo Clinic colleague, hand and microvascular surgeon, Alexander Y. Shin, M.D.: "While free MFC has been used in other difficult nonunions, its use as an interposition vascularized bone graft for scaphoid nonunions with carpal collapse and AVN was pioneered by orthopedic hand surgeons at Mayo Clinic."
The free MFC VBG is ideal in cases of nonunion with AVN and humpback deformity because of the rich vascular supply and the potential for large corticocancellous graft harvest, Dr. Shin explains.
Free MFC is typically indicated when the scaphoid is avascular, as suggested on MRI and confirmed intraoperatively, and when preoperative imaging studies reveal several conditions:
- Carpal collapse determined by a revised carpal height ratio (carpal height divided by capitate length) of 1.52 or less on posteroanterior radiographs, or a radiolunate angle of greater than 15 degrees
- The characteristic fracture of a humpback deformity, the lateral intrascaphoid angle of greater than 45 degrees that is predictive of scaphoid foreshortening
The procedure is a complex microvascular surgical technique that demands expert teamwork. One team harvests the medial femoral condyle graft while the other prepares the scaphoid and recipient vessels. "By mastering this complexity, a promising new technique is emerging for the treatment of scaphoid pathology," Dr. Shin says.
MFC is contraindicated in at least three settings:
- When there is evidence of radioscaphoid arthritis
- In nonunions with normal scaphoid geometry
- In ununited scaphoids with foreshortening that lack evidence of AVN
Osteocartilaginous flap transfers
More recently, Mayo researchers have begun refining an approach that transfers both bone and cartilage grafts from the same location to replace damaged joint surfaces associated with scaphoid proximal pole fragmentation.
Over the years, the free medial femoral condyle (MFC) corticoperiosteal flap has been used in a variety of challenging anatomic contexts for nonunion or bone loss, from the clavicle to orbit. Now, its desirable property of high malleability as a thin flap suits it especially well to scaphoid nonunion pathologies.
"Treatment of fragmented proximal pole is currently evolving, and use of osteocartilaginous free bone flaps is a new alternative," Dr. Bishop says.
Early results of the new technique suggest that using the osteocartilaginous flap as an alternative reconstruction approach is valuable to replace the fragmented scaphoid proximal pole. Notes Dr. Bishop: "Until now, the only other alternative for this situation has been a salvage procedure such as excision of the scaphoid with a 'four-corner,' or lunate-capitate-hamate-triquetrum arthrodesis."