ANATOMY

The subtalar joint is composed of three articulations: the posterior, middle, and anterior joints or facets (Figs. 18-1 and 18-2). Several extra-articular ligaments stabilize the subtalar joint. The major ligaments encountered during subtalar arthroscopy are the intra-articular components, which consist of the interosseous, lateral, and anterior talocalcaneal ligaments. These components coalesce to form the division between the posterior and the middle facets of the subtalar joint. The interosseous ligament is a broad, stout structure that is approximately 2.5 cm wide from its medial to lateral side. It marks the arthroscopic boundary for posterior subtalar arthroscopy.

FIGURE 18-1 Gross dissection of the lateral posterior subtalar joint shows the talus (A), calcaneus (B), and subtalar joint (C).

FIGURE 18-2 A view of the posterior subtalar joint shows the undersurface of the talus (A), the middle facet of the talus (B), the anterior facet of the talus (C), and the superior surface of calcaneus (D).

Open subtalar arthrodesis has historically been the fusion method of choice, and results have generally been favorable. Studies reporting the results of open fusion techniques have all described similar results for fusion rate, time to union, and complications, such as nonunion or malunion, infection, and symptomatic hardware problems.^8–12 Open procedures entail removal of the interosseous ligaments and their vasculature and a lateral incision that can result in nerve dysfunction. Nonunion rates of 5% to 16% have been reported.^4,6 The successful results of arthroscopic ankle fusion have stimulated the development of an arthroscopic approach for subtalar joint arthrodesis, which has the advantages of reduced perioperative morbidity and preservation of the blood supply.^13–15

Arthroscopic subtalar arthrodesis was designed to improve traditional methods by using a minimally invasive technique. Subtalar arthroscopy has been described by several investigators, but few reports of arthroscopic subtalar fusion have been published.¹⁶ The development of an arthroscopic technique for subtalar arthrodesis was intended to lower morbidity using techniques and principles similar to those used for arthroscopic ankle fusion. It was hypothesized that perioperative morbidity could be reduced, blood supply preserved, and proprioceptive and neurosensory input enhanced. These theoretical advantages prompted us to initiate a prospective study to document the effectiveness of the procedure and to determine the time until complete fusion, the incidence of delayed unions and nonunions, and the incidence of complications.

PATIENT EVALUATION

Diagnostic Imaging

Evaluation for subtalar pathology should include plain radiographs of the ankle and subtalar joints. The Broden view provides the best visualization of the posterior subtalar joint and may be sufficient to confirm the diagnosis of subtalar pathology (Fig.18-3). Profound degenerative radiographic changes in the joint are not needed to confirm a diagnosis, because only small alterations in the biomechanics of this joint can produce significant symptoms. Computerized tomography (CT) is best able to define the bony anatomy, and with thin sections, scanning reconstructions in the axial, coronal, and sagittal planes can be performed. Magnetic resonance imaging (MRI) can be used to confirm associated bony changes such as subchondral bone edema and may be useful in evaluating the adjacent soft tissues to rule out other causes of hindfoot pain. Bone scanning and arthrography are usually not required, and these studies have largely been replaced by MRI.

FIGURE 18-3 A Broden view of an arthritic posterior subtalar joint shows the talus (A), the calcaneus (B), and the subtalar joint (C).

TREATMENT

Arthroscopic Technique

Our preferred method begins by placing the patient in the lateral decubitus position and lying on the unaffected side. We place two pillows between the legs while the affected ankle and subtalar joint hang over a blanket roll in a natural position of plantar flexion and neutral version. The patient is prepared and draped, after which the anatomic landmarks and portal sites are identified and marked with a surgical pen. A thigh tourniquet is then inflated.

Establishing the portal sites is one of the more challenging aspects of the procedure. It is critical to predetermine the angles of the subtalar joint, because its unique geometry and limited access requires precise determination to prevent error. Fluoroscopy should be used to confirm portal location if necessary.

The anterolateral and the posterolateral portals are conventionally used. An accessory portal usually needs to be established approximately 1 cm posterior to the anterolateral portal (Fig. 18-4). This portal can be used for debridement or for outflow enhancement and may occasionally be used for visualization. The anterolateral and posterolateral portals are used in an alternating manner for viewing and for instrumentation. If significant arthrofibrosis makes entry and visualization difficult, the accessory anterolateral portal is quite useful.

FIGURE 18-4 Standard portal placement for subtalar arthrodesis includes the anterolateral portal (A), posterolateral portal (B), and accessory lateral portal (C).

A 2.7-mm, wide-angle, small joint arthroscope should be used for this procedure. It should be equipped with a choice of sheaths to accommodate limited or increased flow. The blunt trocar and sheath are introduced through the anterolateral portal. The posterolateral portal can be established at this time, as can an accessory anterolateral portal. In patients who were initially treated with this procedure, a small laminar spreader was used in the anterolateral portal to increase access. This technique is no longer routinely used, but may be useful if subtalar joint distraction is a problem. Arthroscopic resection of the interosseous ligament also may provide additional distraction.

It is important to confirm that the arthroscope is in the subtalar joint and that the ankle joint or the fibular talar recess have not been inadvertently entered. All débridement and decortication is done posterior to the interosseous ligament because only the posterior facet is fused (Fig. 18-5). The middle and anterior facets are not seen under normal circumstances unless the interosseous ligament is absent. Most of the procedure is performed with the arthroscope in the anterolateral portal and the instruments in the posterolateral portal. The remaining débridement is accomplished by alternating the use of these portals.

FIGURE 18-5 An arthroscopic view of the subtalar joint shows the talus (A), calcaneus (B), and posteromedial corner (C).

At the beginning of the procedure, a primary synovectomy and debridement are necessary for visualization, usually through the accessory anterolateral portal (Fig. 18-6). The articular surface is debrided, making the joint more capacious and making the use of instrumentation easier. Complete removal of the articular surface down to subchondral bone is performed next (Figs. 18-7 and 18-8). The unique talocalcaneal geometry requires a variety of instruments, including straight and angled curettes and a complete set of shavers and burrs.

FIGURE 18-6 Electrocautery is used for soft tissue débridement of the subtalar joint. The scope is viewing from the anterolateral portal, and the electrocautery device is working through the posterolateral portal.

FIGURE 18-7 The remaining articular cartilage within the subtalar joint is removed with an arthroscopic burr. The scope is viewing from the posterolateral portal, and the burr is working through the anterolateral portal.

FIGURE 18-8 The subtalar joint is ready for arthrodesis after soft tissue and bony decortication.

After the articular cartilage has been resected, approximately 1 to 2 mm of subchondral bone is removed to expose the highly vascular cancellous bone. Care must be taken not to alter the articular surface geometry and not to remove excessive bone because this will lead to poor coaptation of the joint surfaces. After the subchondral plate is removed, small “spot-weld” holes approximately 2 mm deep are fashioned on the surfaces of the calcaneus and talus to create vascular channels (Fig. 18-9). Careful assessment of the posteromedial corner must be made, because residual bone and cartilage can interfere with talocalcaneal coaptation. The curette often safely breaks down this corner and can provide the surgeon with tactile feedback. The neurovascular bundle is located in the posteromedial corner, and it must be protected throughout the procedure.

FIGURE 18-10 A guidewire is passed through the subtalar joint in an anterior to posterior direction in preparation for screw fixation.

After viewing from both portals to ensure complete débridement and decortication, the tourniquet is released, and the vascularity of the calcaneus and talus is carefully assessed. The joint is then thoroughly irrigated to remove bone fragments and debris. Autogenous bone graft or bone substitute are not needed for this procedure.

The fusion is fixed with a large, cannulated, 7.5-mm screw. The guide pin is started at the dorsal anteromedial talus and angled posterior and inferior to the posterolateral calcaneus; however, it does not violate the calcaneal cortical surface. Using fluoroscopy, the guidewire is placed with the ankle in maximum dorsiflexion to avoid screw head encroachment or impingement on the anterior lip of the tibia (Fig. 18-10). After the guidewire is placed, the ankle can be relaxed, the screw inserted under fluoroscopic control, and the fusion site compressed (Fig. 18-11). The screw runs along the natural axis of rotation of the subtalar joint using this technique. Starting the screw from the dorsal and medial aspect of the talus avoids painful screw head prominence over the calcaneus and reduces the need for a second procedure to remove the screw. There have been no fractures or complications with this particular fixation technique.¹⁵ An alternate method of fixation uses a single screw placed from the calcaneus into the talus in the appropriate direction.

FIGURE 18-9 The superior surface of the calcaneus (A) is shown after decortication and “spot welding” (B).

FIGURE 18-11 A 7.3-mm, partially threaded, cannulated screw is used to achieve fixation across the subtalar joint.

PEARLS& PITFALLS

PEARLS

• Extensive synovectomy may be necessary to establish visualization. Triangulate the arthroscope in the anterolateral portal and a shaver in the accessory portal. Touch the shaver to the shaft of the scope and walk the shaver to the tip of the scope. Shave the synovium to establish the space.

• Have a selection of straight and angled curettes in an assortment of sizes to approach all portions of the articular surface.

• Do not get bogged down in one area. Move the scope and instruments to access all areas of the joint.

• Débride only the subchondral bone to the level of viable bleeding bone, which is documented by deflating the tourniquet and visualizing punctate bleeding. Maintain the normal contours of the joint surfaces to maximize the surface area of joint coaptation.

PITFALLS

• Be certain that the degenerative problem is limited to the subtalar joint by thoroughly reviewing plain radiographs and correlating with a CT or MRI. The presence of significant talonavicular or calcaneocuboid arthritis requires triple arthrodesis rather than isolated subtalar arthrodesis.

• Use differential diagnostic injections into the ankle joint and subtalar joint to ensure that the problem is limited to the subtalar joint.

• Assess for a fixed hindfoot deformity, which may require an osteotomy to realign the hindfoot. Persistent hindfoot deformity results in failure to relieve clinical symptoms and may result in transfer of stress to other parts of the foot and ankle.

RESULTS OF TREATMENT AND COMPLICATIONS

Since September 1992, we have followed 25 consecutive patients undergoing arthroscopic subtalar fusion, and there has been sufficient follow-up time to determine the effectiveness of the procedure. Fusion rate, time until complete union, surgical technique, and the complication rate were analyzed. One standard surgical procedure was used, and the method of internal fixation was consistent throughout this study. The posterior subtalar joint was the only joint fused during this procedure. Three of the 25 patients had a combined arthroscopic ankle and subtalar fusion. In this study, 8 patients had osteoarthritis, 10 had post-traumatic arthritis, 4 had posterior tibial tendon dysfunction, 2 had rheumatoid arthritis, and 1 patient had a talocalcaneal coalition. Every patient had a radiographic evaluation at 2-week intervals to determine the rate and quality of fusion. Clinical and radiographic forms of evidence were required to categorize an arthrodesis as completely fused. The parameters required for a successful arthrodesis are evidence of bone consolidation across the subtalar joint, no motion at the screw, the absence of pain with weight bearing, and pain-free forced inversion and eversion. The mean follow-up time was 22 months (range, 6 to 92 months). The subtalar joint of all 25 patients had united clinically and radiographically; the average time until complete fusion was 8.9 weeks (range, 6 to 16 weeks).¹⁵

There have been no additional operations, with the exception of two screw removals. Two patients had some residual anterolateral pain with some radiographic and clinical evidence of minor degenerative joint disease in the ankle. These findings were documented on preoperative films. One patient had complete pain relief after a diagnostic and therapeutic steroid and lidocaine injection into the ankle joint. One patient eventually underwent arthroscopic ankle arthrodesis because of preexisting osteoarthritis of the ankle. Valgus tilting of the ankle joint after subtalar arthrodesis has been reported, but it is unclear whether this results from the fusion or is merely a natural progression of the disease (TC Fitzgibbons, MD, Dublin, Ireland, personal communication, 1995).

Two patients are not included in this study because the procedure could not be completed arthroscopically because of significant malformation of the calcaneus and arthrofibrosis of the subtalar joint. These patients underwent a modified mini-open posterior subtalar arthrodesis. Identical screw fixation and postoperative protocol was used in these two patients. Skin complications about the hindfoot can be catastrophic and are minimized using the arthroscopic technique. No patients in this study had superficial or deep infections. All arthroscopic procedures have been associated with reduced rates of infection. Those who favor arthroscopic subtalar arthrodesis believe that the same trends seen in other arthroscopic procedures will be demonstrated for the subtalar joint.

COMPARISON OF PROCEDURES

Arthroscopic subtalar fusion has many advantages compared with open procedures. It is a minimally invasive technique that theoretically preserves the blood supply of the calcaneus and talus, which is especially important because many of these patients have had previous open procedures. Conventional open procedures by definition interrupt the blood supply and compromise vascular ingrowth and eventual fusion. Avoidance of incisions coupled with early range of motion and weight bearing helps to avoid stress depravation and enhances proprioception, reducing the devastating effects of complex regional pain syndrome.

Glanzmann conducted a prospective analysis of 41consecutive arthroscopic subtalar arthrodeses. The average modified American Orthopaedic Foot and Ankle Society (AOFAS) ankle-hindfoot score improved from 53 points (range, 22 to 69) preoperatively to 84 points (range, 41 to 94) at the final follow-up assessment (average, 55 months). Union was achieved in all cases.¹⁷ These data echo the results demonstrated by Tasto and colleagues,²² showing reproducible excellent results with arthroscopic subtalar fusion.

In a comparison of open and arthroscopic isolated subtalar arthrodesis, Scranton reported a union rate of 100% in the five arthroscopic procedures and one nonunion in the 17 open procedures.¹² This was a small study with limited statistical power, and a larger, prospective, randomized trial is needed before a direct comparison of open versus arthroscopic subtalar arthrodesis can be made. However, some studies show a trend toward higher fusion rates with the use of arthroscopic procedures. Easley and coworkers⁴ reported a union rate of 84% for 148 patients who had isolated open subtalar arthrodesis, compared with the 100% rate of union in the two arthroscopic studies performed by Tasto and Glanzmann.²² Smoking, the presence of more than 2 mm of avascular bone at the arthrodesis site, and the failure of a previous operative procedure were identified as factors contributing to a less favorable union rate.⁴ Perhaps the observed higher fusion rate in the arthroscopic cohort reflects the preservation of the blood supply and the ability to initiate early weight bearing. One disadvantage of arthroscopic subtalar arthrodesis is that it requires a high level of experience in ankle and subtalar arthroscopy.

In the original technique for arthroscopic subtalar arthrodesis reported by Tasto,²² the arthroscope enters the subtalar joint from the lateral position. This approach has been demonstrated to be safe and reliable.¹⁵ Amendola has added to the literature by describing a posterior arthroscopic subtalar arthrodesis.^18,19

There are advantages to prone positioning. Parisien and Vagsness¹⁴ found that posterior lesions and loose bodies were seen and treated more effectively through two posterior portals when the limb was situated prone. There is concern, however, that the tibial nerve, the posterior tibial artery, and the medial calcaneal nerve are at risk when the posteromedial portal is used.^20–23 Voto and colleagues²³ found that the posteromedial portal is “potentially hazardous” even if placed adjacent to the Achilles tendon. Ferkel²¹ reported a series of 612 ankle arthroscopies with a neurologic complication rate of 4.4%. He recommended against use of anterocentral, trans-Achilles, and posteromedial portals.

Acevedo and coworkers²⁴ demonstrated that coaxial posteromedial and posterolateral portals were safe, effective, and reproducible. Sitler and colleagues,²⁵ in a cadaver study with the limb prone, demonstrated an average distance between the posteromedial portal and the tibial nerve, posterior tibial artery, and medial calcaneal nerve of 6.4 mm, 9.6 mm, and 17.1 mm, respectively. Based on these results, some consider the posteromedial portal to be safe, provided it is kept directly adjacent to the Achilles tendon and the patient is in the prone position.

CONCLUSIONS

Although open subtalar arthrodesis has historically proved to be safe and effective, the arthroscopic approach offers several advantages, including lower morbidity, preservation of the interosseous ligaments and vascularity, shorter hospital stay, early weight bearing, faster rehabilitation, and a better fusion rate. However, arthroscopic subtalar arthrodesis is a technically demanding procedure that requires advanced arthroscopic skills. Subtalar joint access is more difficult than other joints because of the complex, curved articular surfaces held by tight ligaments, and it requires the use of small joint instrumentation. Because deformities cannot be corrected using this arthroscopic technique, the procedure must be considered only for fusion in situ. There are relatively few patients who qualify for this procedure, and the learning curve for improving surgical skills is steep. Cadaveric surgical skills workshops are the most efficacious method of mastering this technique.

The results of arthroscopic subtalar arthrodesis appear to be excellent in terms of patient satisfaction, fusion rate, time until union, and postoperative morbidity. Continued research is needed to compare open and arthroscopic techniques, determine the safest and most reliable arthroscopic approach, and promote continuing education to help surgeons master these important skills.