Fig. 7.1
Schon’s clinical stages of the degree of deformity are based on physical examination. In stage (b), the midtarsus is coplanar with the metatarsocalcaneal plane. In stage (a), the midtarsus is above this plane. In stage (c), the midtarsus is below this plane (From Schon et al. [5])
Radiographic classification systems have also been developed by Schon et al. as well as Sammarco and Conti in an effort to better understand which patterns of dislocation are more likely to require surgery (Fig. 7.2) [6, 7]. It is our experience that it is not the pattern of deformity, but rather the severity of deformity which predicts the necessity for surgery. Schon’s group defined deformity severity as “alpha” or “beta.” Stage beta indicates more severe deformity and is assigned if one or more of the following criteria are met: (1) a dislocation is present, (2) the lateral talar – first metatarsal angle is ≥30°, (3) the lateral calcaneal-fifth metatarsal angle is ≥0°, or (4) the AP talar-first metatarsal angle is ≥35°. Sammarco and Conti quantified the severity of deformity by the measured amount of dorsal translation of the forefoot through the midfoot (Fig. 7.3).
Fig. 7.2
Schon’s classification of diabetic midtarsus deformity is based on the anatomic area of involvement: type I, Lisfranc pattern; type II, naviculocuneiform pattern; type III, perinavicular pattern; type IV, transverse tarsal (Chopart) pattern (From Schon et al. [5])
Fig. 7.3
As described by Sammarco et al., the amount of dorsal midfoot displacement is the vertical distance measured between the midline of the lateral talar line at the level of dislocation (point B) and the midline of the lateral first metatarsal axis (point A) on weight-bearing radiographs (From Sammarco et al. [11])
My indications for midfoot arthrodesis in a patient with a neuroarthropathic midfoot dislocation are:
- 1.
Chronic stable foot with significant deformity (Schon clinical Grade C) with recurrent ulceration despite appropriate brace management
- 2.
Chronic unstable foot deformity which has failed conservative management
- 3.
Acute or subacute gross instability with severe deformity (i.e., a Schon beta deformity or 1 cm midfoot dislocation in the Sammarco schema)
Contraindications
The importance of medical optimization prior to surgical intervention cannot be overstressed. Appropriate control of blood glucose is necessary for wound healing and osseous consolidation of the arthrodesis site. Adequate vascularity must be established prior to any surgical intervention, and if there is a question of adequate arterial supply to the foot, then further investigation is warranted. Vascularity can be assessed by segmental arterial pressure readings (which should include toe measurements) and also by skin oxygen perfusion studies when available. In the absence of good arterial supply, surgery should be deferred until adequate arterial supply is obtained by endovascular or surgical techniques. If an adequate vascularity cannot be established, the patient may be better served by amputation of the limb or nonsurgical treatment.
The presence or absence of deep infection must also be established at the outset of the limb salvage process. A detailed discussion of this process is beyond the scope of this text, and the reader is referred for further reading [8, 9]. Ulcers that do not extend deeply into the foot should be allowed to heal by conservative management such as total contact casting prior to proceeding with fusion. A non-healing ulcer in a well-vascularized limb that extends to soft bone is presumed to be osteomyelitis. If osteomyelitis is present, limb salvage may still be possible, but must be staged in order to first resolve the infectious process prior to implanting permanent orthopedic hardware. This usually requires surgical debridement and application of an external fixator to off-load the limb and reduce and stabilize the dislocation.
Pearls and Pitfalls
Deformity Correction
Reduction of the deformity can be technically challenging and requires adequate soft tissue release combined with aggressive bone resection. Attempts at reducing the foot without adequate bone resection can place significant strain on the neurologic and vascular structures of the foot, which contract with the remainder of the soft tissue envelope during the disease process. Bone resection is therefore necessary to allow for reduction without undue tension on the soft tissue envelope.
Aggressive surgical correction of equinus is necessary both in achieving correction of the midfoot deformity and preventing recurrence following correction.
It is helpful to conceptualize the foot as two blocks of bone rather than multiple individual osseous structures. Once the deformity is exposed subperiosteally at the level of the dislocation, bone is resected to allow realignment of the metatarsals with the talus and calcaneus. Typically the forefoot dislocates dorsally and laterally, and a wedged bone resection is performed more medially and plantarly. Often dense scar tissue and contracted ligamentous structures are present dorsally which must be released and resected.
Biologic Challenges and Fixation
Certain surgical techniques can contribute to successful reconstruction of the neuroarthropathic foot. Poor bone quality, osteoporosis, and poor osseous vascularity in the area of intended fixation are often present. Dissolution of bone can accompany the hypervascularity present during the acute Charcot process. Subsequent fracture and fragmentation often lead to fibrosis and impaired vascularity of the osseous remnants that are present after consolidation. All of these factors make fixation a challenge and can lead to longer healing times. Obesity and poor compliance with weight-bearing restrictions may also jeopardize the outcome.
Modern fixation techniques for Charcot attempt to span the area of dissolution and achieve fixation in proximal and distal bone that is less affected by the disease process. We introduced the concept of the “superconstruct” for surgical reconstruction of the neuroarthropathic foot to address these issues [8]. A superconstruct is defined by four factors: (1) fusion is extended beyond the zone of injury to include healthy joints in order to improve fixation, (2) bone resection is performed to shorten the limb to allow for adequate reduction of deformity without undue tension on the soft tissue envelope, (3) the strongest implant that can be tolerated by the soft tissue envelope is used, and (4) the implants are positioned to maximize mechanical stability. Current superconstruct techniques include the use of plates applied on the plantar aspect of the foot (plantar plating technique) and the use of screws applied axially through the foot spanning into the intramedullary canals of the metatarsals (axial screw technique).
Approaches and Techniques
Plantar Plating Technique
Plate fixation in the midfoot offers significant mechanical advantages when compared to the use of screws, Kirschner wires, or staple fixation. Plating allows fixation to be extended beyond the zone of fragmentation and dissolution in the neuropathic foot and into the denser cortical bone of the metatarsals. Technically, plate fixation is more straightforward when the fixation device is applied dorsally and/or medially. However, Schon developed the concept of plantar plating noting that the plantar location places the device on the tension side of the deformity [10]. With weight bearing, compression occurs at the fusion site (Fig. 7.4).
Fig. 7.4
Plantar plating . (a and b) Preoperative and postoperative X-rays of a patient with fracture dislocation though the midfoot treated by midfoot arthrodesis fixed with plantar plates. (c) Biomechanically, the plate is under tension with weight bearing which generates compression through the fusion mass (From Pinzur et al. [15], American Academy of Orthopedic Surgeons)
Axial Screw Technique
Intraosseous fixation of the midfoot can be achieved by passing long screws through the metatarsal shafts distally to span the area of Charcot dissolution, achieving fixation proximally in good quality bone [11]. The screws can be applied antegrade from the calcaneus or talus or retrograde through the metatarsophalangeal joints. Open surgical preparation of the midfoot joints at the level of the deformity is done and includes resection of articular cartilage and degenerative bone. However, it is not necessary to prepare all of the joints bridged by the fixation screws for arthrodesis.
The axial screw technique has several advantages. Placement and positioning of the screws aid in reduction of the deformity. Using guide wires for cannulated screws allows for provisional fixation and the foot to be held in the desired corrected position while being confirmed with radiographic imaging. Redirecting the guide wires is relatively simple, and once the deformity is reduced and held with wires, applying the screws directly over the wires completes the fixation. Compression of the arthrodesis sites can be achieved by simply tightening the screws. The intramedullary positioning of the screws minimizes stress risers in the shafts of the metatarsals which can occur from transcortical screws. The axial screw technique also respects the osseous blood supply since the screws are applied through more limited incisions and without the extensive stripping of bone necessary for application of plates. The intraosseous position of the screw diminishes concern for exposed hardware in the event of a wound failure or ulceration. Once in place, the axial screws act as load-sharing devices with the existing osseous structures (Fig. 7.5).
