The chapter starts out with the history of the isolated subtalar fusion and the evolution to its today’s presentation accompanied by general and biomechanical considerations. Indications and techniques for the open in-situ and open distraction fusion, as well as for the arthroscopic fusion, are described in detail. Presenting the postoperative outcome, the entire literature of the past two decades is included.
During removal of the cartilage, care is taken to maintain the normal bone contour of the joint and not to remove the subchondral bone, as bony quality for compression is impaired.
To prevent non-union, nonviable bone is removed, and the subchondral bone is drilled and roughened (fish-scaling).
Only an appropriate visualization into the subtalar joint allows a complete preparation of the fusion site.
The correct heel positioning is crucial for good outcomes.
Careful preparation is mandatory at the posteromedial corner of the posterior facet (neurovascular bundle, flexor hallucis tendon) and using the posterolateral approach for distraction fusion (sural nerve). It is helpful to ensure that the arthritis is isolated to the subtalar joint by differential injections or computed tomography. Realignment of the hindfoot may change the forefoot position.
Occasionally, additional correction of the forefoot is required. To avoid misplaced screws, the positioning is checked intraoperatively with image intensifier in four views.
HISTORY/INTRODUCTION/SCOPE OF THE PROBLEM
An isolated open subtalar fusion was first described by W. E. Gallie in 1943 for posttraumatic subtalar arthritis through a posterolateral approach. Tibial bone graft was driven into a preformed trapezoid slot between calcaneus and talus. No implants were used for fixation and compression of the fusion site. Kalamchi modified Gallie’s procedure by harvesting the outer half of the posttraumatically broadened calcaneus as donor site for the bone graft. The Grice extra-articular subtalar arthrodesis was originally introduced for paralytic children’s flatfeet in 1952 and spared the joint surfaces to prevent interference with the growth of the foot. The technique consisted of the placement of two tibial cortical bone grafts into the sinus tarsi. The indication of this technique was expanded to adult patients in the late 1950s and the 1960s.
At the same time it has been suggested that the abolition of subtalar motion after fusion increases the stress in the adjacent transversal tarsal joints leading to important degenerative changes. Therefore, the triple arthrodesis for subtalar arthritis was favored traditionally for the operative treatment of talocalcaneal problems. However, in the past three decades many authors stated that arthritis of the transverse tarsal joints are of little clinical significance, less frequent than described before, and good results for isolated subtalar fusion are reported.
Compared with the triple arthrodesis, advantages of an isolated subtalar fusion are the partially preserved hindfoot motion and therefore a lower risk of arthritis of adjacent joints, a less complex operative procedure with lower rate of infection and avascular necrosis, and elimination of the risk of non-union or malunion of the transverse tarsal joints. If clinically significant degenerative changes occur after subtalar fusion, a triple arthrodesis still can be done at that time.
In biomechanical investigations, Wülker et al. revealed that a subtalar fusion decreases the extent of dorsiflexion/plantarflexion at 55% and supination/pronation at 70% in the talonavicular joint. Changes were statistically significant. However, the reduction of hindfoot motion was more restricted after isolated fusion of the talonavicular joint, and the fusion of the calcaneocuboid had only minor influence on the overall hindfoot motion.
Astion et al. noticed in a cadaver model an overall decrease of 26% of talonavicular motion and of 56% of calcaneocuboid motion after simulated subtalar fusion.
The function of the subtalar joint is closely interrelated to the talonavicular and calcaneocuboid joint. A motion at the subtalar joint indispensably results in a motion at the transversal tarsal joint line. If the physiologic interactions in this functional unit are altered by a subtalar fusion, the function of the remaining joints in hindfoot is potentially impaired leading to arthritis of the adjacent joints.
Whenever feasible, the interactions in this functional unit are restored in case of neutral hindfoot alignment and are corrected in case of hindfoot deformity or posttraumatic condition. Even in neutral hindfoot alignment the loss of height after removal of the cartilage for the subtalar fusion may disturb the talonavicular and calcaneocuboid joint’s interactions. Therefore, bone grafting to prevent arthritis of the adjacent joints, as well as to improve healing of the fusion site, is advocated by some authors.
The basic indication for an isolated fusion of the subtalar joint is advanced osteoarthritis once nonoperative treatment has failed. The subtalar fusion is part of triple or pantalar fusions, or is combined with talonavicular and calcaneocuboid fusions. In combination with a replacement of the talonavicular joint, an arthroplasty of the subtalar joint has been described, but it has never become widely accepted.
The commonest indication (41% to 73% of cases) for isolated subtalar fusion is the posttraumatic subtalar arthritis after calcaneal fracture. Further indications are primary joint degeneration, adjacent subtalar arthritis after ankle joint arthrodesis, postinfectious conditions, talocalcaneal coalitions, ligamentous instabilities, and hindfoot valgus deformities or—very rarely—hindfoot varus deformities in neurologic disorders.
In rheumatoid arthritis, an isolated subtalar fusion is rarely indicated because of arthritic change in the rest of the foot and is therefore combined with further hindfoot and/or mid- and forefoot procedures. The poor bone quality in some rheumatoid patients may affect the surgeon’s ability to achieve stable internal fixation of the fusion.
The patient typically complains about weight-bearing pain localized beneath or under the lateral and medial malleolus and stiffness in the hindfoot. The discomfort is worse walking on uneven ground, which forces the heel to invert and evert. On the lateral side in particular, the clinical examination reveals tenderness along the subtalar joint line and a moderate to severely restricted range of motion of the subtalar joint is seen compared with the healthy side. A hindfoot deformity may be obvious but the amount should be estimated examining the standing patient from behind and at heel rise. The ligamentous hindfoot stability needs to be assessed, too. In posttraumatic or valgus hindfoot deformities the lateral pain may be caused by impingement of the lateral calcaneal wall at the fibular tip. Impingement may also occur in the planovalgus foot between the anterior process of the talus and the dorsal side of the calcaneus in the sinus tarsi. This pain should be differentiated from subtalar pain as this patient will benefit from an arch reconstruction more than a subtalar fusion if subtalar motion is still present.
Often plain-standing radiographs are difficult to interpret regarding the amount of subtalar arthritis as the narrowed joint line is not clearly visible on the standard lateral view, particularly in valgus or varus hindfoot deformities. A computed tomography (CT) scan is useful, and with a diagnostic injection of local anesthesia (and cortisone) the pain relief after solid fusion can be estimated. A conclusive injection is performed with contrast under image intensifier guidance.
Prior to isolated subtalar fusion, osteoarthritis of the adjacent joints is excluded clinically and radiographically. Because a calcaneocuboid fusion has only minor influence on the overall motion of the hindfoot, a simultaneous fusion may also be considered if the calcaneocuboid joint demonstrates advanced arthritic changes, too.
Open Subtalar In-Situ Fusion
The patient is placed in a lateral or semilateral position. Perioperative antibiotics and the use of a tourniquet are recommended. The standard approach for isolated open in-situ subtalar fusion at present is the transverse lateral subtalar approach centered over the sinus tarsi ( Fig. 26-1 ). The skin is incised from the distal fibular tip to the cuboid, in line with the fourth ray. The superficial peroneal nerve and the sural nerve may cross the incision as they are variable in their anatomy and connecting rami between the two nerves may be present. The origin of the extensor digiti brevis muscle is split or partially detached. Posteriorly the subtalar approach to the joint is limited by the fibulocalcaneal ligament and the peroneal tendons. Anteriorly, the approach does not need to exceed the calcaneocuboid joint. The sinus tarsi fat pad is incised directly superior to the peroneal tendon sheath and should be preserved for anatomic closure by lifting it up. The posterior and the anteromedial facets are exposed by resecting the interosseous ligament and by inversion of the calcaneus. If the laminar spreader is used, it is placed in the sinus tarsi to avoid fracture of the talus or calcaneus. During debridement and removal of the cartilage, care is taken to maintain the normal bone contour of the joint and not to remove the subchondral bone. The bony resistance to the compression applied with the screw insertions may be impaired. To prevent non-union nonviable bone is removed. For the removal of the cartilage straight and curved osteotomies and curettes are used. The subchondral bone is drilled with 2.0 and/or roughened with osteotomies (fish-scaling). Care should be taken not to damage the neurovascular bundle and the tendon of the flexor hallucis longus in the posteromedial corner of the posterior facet. A curved curette working toward the surgeon is preferable to an osteotome working medially in this corner.
The heel position is crucial in subtalar fusion and may influence the functional outcome a lot. As the hindfoot is supple in the valgus position, and a supple hindfoot can absorb more impact energy than a rigid one, neutral alignment to 10 degrees of hindfoot valgus is the recommended positioning the subtalar fusion. The hindfoot position is adjusted to the contralateral healthy side on the basis of clinical preoperative or intraoperative assessment and on the use of intraoperative image intensifier for anteroposterior views of the hindfoot.
Compression across the fusion site is desired in in-situ subtalar fusion. Usually two cannulated or regular posterior-to-anterior 6.5-, 7.0-, or 7.3-mm screws are recommended for the fusion’s fixation ( Fig. 26-2 ). Both fully threaded lag screws and partially threaded screws with short 16-mm or 32-mm thread are placed from the non–weight-bearing upper part of the tuber calcanei into the talar body or in the talar body and talar head. If partially threaded screws are used, it is imperative that all threads be across the subtalar joint to ensure compression. An anterior cruciate ligament drill guide was recommended to direct the guide pin to the desired position. The use of a washer in the primary subtalar fusion is rarely required but may be indicated in revision surgery or rheumatoid patients.
Alternatively to the standard screw position, a second screw can be inserted from a lateral-plantar stab incision through the anterior calcaneal process into the talar head. If exceptionally the talar bony quality is worse than the calcaneal quality, one or two anterior-to-posterior screws may provide improved bony purchase in the calcaneus. Therefore, 32-mm threaded 7.0-mm cannulated screws are fixed from the talar neck into the calcaneus; however, the placement is more difficult as the anterior neurovascular bundle is close to the insertion area. Usually the talus provides by far a better bony purchase than the calcaneus. The central body of the calcaneus has minimal bone and should be avoided with a talus to calcaneus screw if good compression is required. An alternative plantar-to-superior screw position anterior to the calcaneal tuberosity was found to provide biomechanical bony purchase equal to the standard posterior-to-anterior position and may be useful for revisions of failed subtalar fusion. Two screws instead of one provide more rigid fixation and prevent rotatory movements around the axis of subtalar motion. A single screw may therefore increase the chance of non-union compared with two screw fixation. A literature survey shows that about half of the authors use one screw for any subtalar fusion. Most authors used partially threaded cannulated 7.0/7.5-mm screws.
The appropriate positioning of the screws is checked with the image intensifier to avoid intra-articular or extraosseous implants. The following views should be taken: the anteroposterior view of the foot ensures that the screws are in the talar head. The anteroposterior view of the ankle ensures that the screws are not in the lateral gutter. The lateral view of the foot ensures that the screw heads are not sticking out at the calcaneal tuberosity and are not inserted too plantarly, that the threaded part is in the talus body, and that the tip is not extraosseous anteriorly, and the axial calcaneus view ensures that there is no medial extraosseous positioning.
After the fusion site is fixed, the wound is irrigated, followed by reapproximation of the extensor digitorum brevis muscle and the sinus tarsi fat pad. The skin is closed in layers.
The in-situ subtalar fusion cannot, however, restore heel height, the talocalcaneal angle, or the talar declination angle in deformities that involve collapse of the subtalar joint. In these cases, subtalar bone block distraction fusion is advised to restore satisfactory orientation of the hindfoot and ankle.
Open Distraction Subtalar Fusion
In 1988, Carr et al. published the subtalar bone block fusion as a procedure to restore the anatomical relationships of tibiotalar and subtalar joints in sequelae of previous calcaneal fractures. A similar procedure may be required for patients with tarsal coalitions. The hindfoot in a collapsed posttraumatic calcaneus presents with an incongruous subtalar joint, a decreased calcaneal body height with functional impairment of the gastrocnemius–soleus complex, anterior tibiotalar impingement, a lateral calcaneal wall expansion with calcaneal-fibular abutment and peroneal tendon impingement, usually a hindfoot varus, midfoot malalignment, and flattening of the longitudinal arch. Indication for the bone block distraction was considered be loss of heel height of more than 8 mm and radiologic evidence of anterior tibiotalar impingement. Postoperative pain relief is closely related to an appropriate lateral decompression of peroneal/lateral wall impingement.
The patient is placed in a prone, semilateral, or even lateral position. Predominantly, the longitudinal posterior approach is recommended, but some authors prefer the transverse anterolateral approach. However, wound breakdown after open distraction subtalar fusion is more likely to occur in a transverse than in a longitudinal incision as the heel’s soft tissues are lengthened, and the tricortical bone graft is easier inserted through the posterolateral approach.
The posterior approach includes a straight incision parallel and slightly lateral to the Achilles tendon. The sural nerve is identified and preserved, if possible. If appropriate exposure is achieved only with excessive traction or even damage of the sural nerve, it is recommended to transect the nerve and to bury it in the flexor hallucis longus muscle to prevent painful neuromas. If calcaneal-fibular abutment and peroneal tendon impingement are present, the lateral wall of the calcaneus is removed partially to normal width. Decompression of the peroneal tendons prevents ongoing lateral discomfort. The subtalar joint is identified and opened posteriorly. Again, care should be taken to preserve the medial neurovascular bundle and tendons. However, the realignment of severe deformities in distraction subtalar fusion usually requires a medial and lateral capsular release for appropriate correction. A medial incision may be required to release the medial capsule and allow placement of graft on the medial side of the joint.
If the release is not complete, the deformity is potentially made worse. The remaining cartilage is curetted and a laminar spreader is then placed in the joint to distract it open. The subchondral bone is drilled with 2.0 and/or roughened with osteotomies (fish-scaling). To anticipate a hindfoot varus, the spreader is placed as far medially as possible. If the laminar spreader is too weak to distract the joint appropriately or if bony surface is impressed, a femoral distractor is used alternatively. Appropriate distraction is ensured with the image intensifier. A tricortical bone block taken from the iliac crest is then positioned while holding the hindfoot distracted and in slight valgus ( Fig. 26-3 ). The amount of required distraction is based on the loss of heel height as calculated by the preoperative computed tomography scan and radiographs. In sequelae of calcaneal fractures with significant hindfoot shortening and loss of height, one or two angulated tricortical bone blocks of 10- to 15-mm height at their basis are harvested from the iliac crest. Additional cancellous bone graft may be added. The fusion is fixed as described earlier.