How To Address Avascular Necrosis

When surgeons perform distal first metatarsal osteotomies, avascular necrosis (AVN) is a relatively common complication. Accordingly, these authors emphasize having a strong understanding of the involved vascular anatomy, share intraoperative pearls for minimizing the risk and offer treatment insights when AVN does occur.

     Although it is a relatively rare problem, avascular necrosis (AVN) of the first metatarsal head is a condition that we should be aware of and understand. This condition started getting attention when surgeons offered the first descriptions of distal metatarsal osteotomies for the correction of hallux valgus (HAV) deformities.1 These procedures have become a staple for the correction of mild to moderate HAV deformities and generally have a high success rate.

     Avascular necrosis may occur in multiple bones throughout the body. In the foot, it most commonly affects the talus, first and second metatarsals, and the navicular and sesamoid bones. For the purposes of this article, we will offer a closer look at the development of AVN in the first metatarsal head and discuss proactive methods to minimize the risk of this complication.

     Avascular necrosis is bone death secondary to ischemia. When blood supply to the bone is disrupted, it deprives the cells of oxygen and leads to osteonecrosis. This typically progresses as a series of events that begin with an initial insult and eventually leads to a loss of the blood supply.

     The body attempts to repair the damaged bone via revascularization, resorption of necrotic bone and reossification. In the process, mechanical failure may occur at the repair interface, leading to subchondral fracturing and possible joint collapse. In this case, cartilaginous destruction takes place, leading to arthrosis and pain. 2

     There are a variety of possible etiologies for AVN. These etiologies include: systemic and injectable corticosteroid use, hemoglobinopathies, Cushing’s disease, alcoholism, complications of trauma, and iatrogenic causes. The most commonly described cases of AVN occur after hallux valgus corrections by way of distal metatarsal osteotomies with or without the use of a lateral soft tissue release.

Keys To Understanding The Vascular Anatomy

     A distal osteotomy disrupts the intraosseous blood supply to the metatarsal head. Given this reality, physicians expressed concern that the surgery may create AVN, especially when one combines the surgery with a lateral capsular release. There have been reports to support this concern. 3 However, better comprehension of the vascular anatomy illuminates safe zones that allow us to perform the surgery with a relatively low rate of complication.

     A thorough understanding of the vascular network is an important first step toward a successful surgery. There are three main arteries that supply the first metatarsal. 4 The first dorsal metatarsal artery is a direct extension of the dorsalis pedis artery and passes dorsal to the first interosseous muscle. Its first branch is the nutrient artery, which enters the proximal one-third of the first metatarsal on the lateral aspect. The first dorsal metatarsal artery continues distally, forming a significant extracapsular anastomosis that supplies approximately two-thirds of the metatarsal head.

     Be advised that careless use of a saw can put this structure at risk when one is making a distal osteotomy. A common mistake is to let the saw blade plunge into the intermetatarsal space as it penetrates the lateral cortex.

     The first plantar metatarsal artery is a direct extension of the deep perforating branch of the dorsalis pedis artery. It travels distally and approaches the first metatarsal head by passing between the heads of the flexor hallucis brevis. This spatial orientation becomes important when the surgeon is performing an adductor hallucis release as part of the lateral capsular release. With careful dissection, the vessel is not at risk of disruption and forms an important capsular anastomosis that supplies the plantar one-third of the metatarsal head.

     Surgeons may frequently encounter these small vessels on the plantar aspect of the metatarsal head when attempting to release capsular tissues. When one sees these small vessels, avoid cauterization and try to leave them intact. Preserving these vessels maximizes circulation to the bone and joint capsule.

     The last vessel supplying the head of the first metatarsal is the superficial branch of the medial plantar artery. The medial plantar artery is a direct extension of the posterior tibial artery and gives rise to deep and superficial branches. It travels on the plantar medial aspect of the first metatarsal and is the smallest of the three main vessels that supply the bone. It gives off branches to help feed the capsular anastomosis and mainly supports the plantar medial aspect of the metatarsal head.

     The first metatarsal head has three primary vascular sources. 5 The first and most important is the interosseous circulation. This is fed by the nutrient artery, which extends from the first dorsal metatarsal artery. The second source comes from the joint capsule. Perforating vessels extend from the capsule into the metatarsal head with the majority coming from the lateral aspect.

     The three aforementioned vessels extend distally to form an anastomosis around the capsule and allow it to feed the bone. The last source of blood to the metatarsal head comes via the periosteum. It does not play as large a role as other long bones in the body but definitely contributes.

Distal First Metatarsal Osteotomy And AVN: What The Literature Reveals

     Since the distal first metatarsal osteotomy was first described, there has been concern over the development of AVN. Mitchell felt that the lateral capsular attachments were the only remaining blood supply to the head of the bone following disruption of the intraosseous blood supply via osteotomy. 6

     Due to this concern, surgeons did not uniformly perform the lateral capsular release for fear of catastrophic results. In 1985, Meier described a series that supported this philosophy by reporting an AVN rate of 20 percent with osteotomy alone in comparison to 40 percent when surgeons combined the osteotomy with a lateral release. 3

     It is quite obvious to any foot and ankle surgeon that the intraosseous blood supply is disrupted during the osteotomy portion of this procedure. The important question is whether it is a permanent disruption or does the vascularity get re-established?

     Resch, et al., looked at this in 1992 with 99mTc scintigraphy scans. 7 A total of four feet showed vascular compromise at two to nine days following a distal osteotomy with all of them resolving on repeat scan at two to four weeks. Interestingly, of the four patients with temporary vascular compromise, one had a lateral release and three had an osteotomy alone.

     Wilkinson, et al., used MRI to evaluate the vascularity to the metatarsal head after a distal osteotomy. 8 According to the study, 50 percent of the patients demonstrated vascular insult following the procedure but all resolved without clinical evidence of AVN.

     Another concern historically was that the lateral capsular attachments were the only remaining blood supply to the head of the bone following a distal osteotomy. Jones and colleagues did an excellent job showing the complex and delicate vascular network that the joint capsule provides to the metatarsal head. 5 The lateral aspect does account for two-thirds of the supply but there is a strong influence from the plantar, medial and dorsal aspects as well. This work highlights safe zones for approaching the lateral release. The authors suggest that combining the lateral release with a distal osteotomy is safe when one uses proper technique.

     In 1993, Green and colleagues performed 164 bunionectomies, combining a lateral release with distal osteotomy. They reported no incidence of AVN. 9 In 1994, Peterson, et al., published a similar study with 58 procedures. 10 One patient developed AVN but was asymptomatic at 4.2 years. More recently, Marks and colleagues looked at 40 patients who underwent a distal first metatarsal ostetomy with lateral release. They noted one asymptomatic case of AVN. 11

A Guide To Pertinent Clinical Findings

     In the first four months after surgery, it can be difficult to differentiate between the early stages of AVN and normal postoperative findings. 12 One will usually encounter atypical edema and increased tenderness around the joint. Since these are similar to normal postoperative findings, experience in this case is helpful and physicians should have a high index of clinical suspicion. You may also note erythema and warmth. One should distinguish these findings from the possibility of an underlying infection. Lastly, early transfer metatarsalgia can be an indicator of an abnormal process in the first metatarsophalangeal joint.

     The later stages of AVN are associated with bone breakdown and arthrosis. In severe cases, there can be visible shortening of the ray. More commonly, however, physicians will encounter pain and stiffness in the joint. 13

Essential Insights On Diagnostic Imaging

     Early radiographic findings can be very subtle following a distal first metatarsal osteotomy. While one will often observe changes, they resolve in the majority of cases without the formation of AVN. Physicians have theorized that these transient radiographic changes represent an adjustment period as the metatarsal head recovers from the initial vascular compromise of the surgery. 12

     Standard radiographs can be misleading and difficult to interpret. 14 The sesamoid bones, which are hopefully well aligned after an appropriate bunion correction, tend to make visualization of the head difficult and can mask subtle changes taking place in the bone. Increased radiodensity and mild resorption medially are common postoperative findings that physicians see on plain films.

     Additionally, one may see cyst formation in 5 percent of normally healing distal first metatarsal osteotomies. If you suspect AVN, obtain serial radiographs for close observation.

     Early stages of AVN show crescent-shaped subchondral lucencies. 15 The head will often demonstrate mottling changes and possible focal cyst formation. As AVN progresses into the later stages, one can see joint space narrowing with subchondral collapse. In addition, fragmentation of the head may take place, leading to degeneration. Until one sees these advanced changes, physicians should not make a definitive diagnosis of AVN.

     Advanced imaging can be helpful when radiographs are inconclusive and you have a high index of suspicion for AVN. Scintigraphy will show changes in the bone but it is non-specific and will react to the initial osteotomy. Resch reported seeing changes consistent with vascular compromise in four patients following distal osteotomy with all of them resolving at two to four weeks. 7 If you are considering scintigraphy, it is recommended to wait at least four weeks in order to avoid confusing the initial vascular insult of surgery with AVN.

     Researchers have shown that MRI is the most sensitive technique for detecting AVN of the talus. 16 One could therefore surmise anecdotally that the same applies for the first metatarsal head. Physicians should utilize MRI when there is a high clinical suspicion for AVN in the setting of a normal or obscured radiograph.

     T1-weighted MRI images typically show decreased signal intensity corresponding to diminished cancellous bone content. Early T2-weighted images show increased signal intensity representing hyperemia and edema, which outline hypointense islands of avascularity.

Treating AVN: What You Should Know

     There is no standard approach to managing first metatarsal head AVN and it is sparsely described in the literature. There are no outcome studies, which most likely is due to the rare nature of the problem. Articles that describe it usually take the form of anecdotal case studies that describe an author’s approach to the problem and how the patient fared clinically. Regardless, most literature advocates conservative therapy progressing to surgical management as needed.

     Early stages of AVN are usually self-resolving and rarely become symptomatic in the long-term. In terms of initial conservative treatment, one may emphasize activity modifications that limit motion in the first MPJ and protect the first metatarsal head. Shoe modifications and/or orthotics can be beneficial in this regard as well. Rest, ice and NSAIDs can also be part of the treatment approach. Physicians should consider partial or complete non-weightbearing when necessary.

     Management of symptomatic AVN in the latter stages becomes more difficult and again is not adequately described. Conservative therapy includes the use of NSAIDs, cortisone injections, physical therapy, shoe modifications and/or orthotics. Symptomatic patients who fail conservative therapy typically move on to surgical intervention.

     Authors have described multiple procedures for AVN with varying degrees of success. These procedures include simple debridement, subchondral drilling, arthroplasty, decompression osteotomy and arthrodesis. 17

     Out of these options, arthrodesis has become popular and most authors have reported good results. 18,19 It has the advantage of removing non-viable cartilage and bone while creating a stable platform for ambulation. This is the procedure of choice when dealing with metatarsal head collapse.

     If the surgeon encounters significant bone loss, he or she should add a block of autogenous bone graft to maintain length of the first ray and prevent transfer metatarsalgia. Be sure to debride all non-viable bone and employ stable fixation to maximize the healing potential.

Final Notes

     Distal first metatarsal osteotomies are one of the most commonly used techniques for repair of hallux valgus deformities. They have been shown to be safe and reliable procedures with excellent clinical results. Although AVN of the first metatarsal head is rare overall, one may see this with distal first metatarsal osteotomies. It is easily avoidable, however, with proper understanding of the anatomy and good surgical technique.

     Disruption of the intraosseous blood supply is unavoidable when one is making a distal osteotomy. Surgeons should take care during this maneuver to avoid damaging vital vessels by not puncturing the lateral capsule or plunging the blade deep into the first intermetatarsal space. Lateral capsular and adductor tendon releases are important adjuncts to these procedures as they allow for better correction and higher patient satisfaction. When one perform these procedures properly, they are safe, can prevent damaging important vascular structures and have minimal increased risk for AVN. 5,10

     The primary mistake leading to AVN is aggressive capsular stripping. The lateral capsular vessels provide the greatest contribution to the metatarsal head and one should accordingly leave these intact. Excess dorsal and plantar release is also poor technique and one should avoid this if possible.

     Dr. Schroeder is a Fellow of the American College of Foot and Ankle Surgeons. He is board certified in foot and ankle surgery as well as reconstructive foot and ankle surgery. Dr. Schroeder is the Chief of Podiatric Surgery at Southwest Washington Medical Center in Vancouver, Wash. He practices at the Vancouver Clinic.

     Dr. Dockery is a Fellow of the American College of Foot and Ankle Surgeons, the American College of Foot and Ankle Pediatrics, and the American Society of Podiatric Dermatology. He is board certified in foot and ankle surgery. Dr. Dockery is the Chairman of the Board and Director of Scientific Affairs for the Northwest Podiatric Foundation for Education & Research, USA. Dr. Dockery is the author of Cutaneous Disorders of the Lower Extremity (Saunders, 1997) and Lower Extremity Soft Tissue & Cutaneous Plastic Surgery (Elsevier Sciences, 2006).

For further reading or to get reprint information, visit www.podiatrytoday.com.

References:

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