Arthrodesis — Part II : The tarsus

Part I of this topic dealt with indications, procedures, and results of carpal arthrodesis in the dog and cat. Almost as frequent are traumatic or degenerative conditions requiring arthrodesis in the tarsus. The surgeon is confronted with 3 common tarsal problems that may require arthrodesis: tarsometatarsal hyperextension or luxation, calcaneoquartal (at the articulation between the calcaneus and 4th tarsal bone) hyperextension or luxation, and tarsocrural trauma. Tarsocrural injury is most often a result of shearing trauma in automobile accidents. While these injuries can be severe, they rarely require arthrodesis. Management of these injuries has been dealt with previously (Can Vet J 2000;41:940–943) and will not be reviewed here.

Intertarsal instability, for all practical purposes, means laxity between the calcaneus and the 4th tarsal bone. While instability can be seen between the talus and the central tarsal bone, it nearly always accompanies calcaneoquartal instability (1,2). In the distal row of tarsal bones, the large 4th tarsal bone provides buttress support medially to the smaller central tarsal bone and tarsal bones I, II, and III, making instability at this level unlikely (3). Calcaneoquartal instability may be traumatic, with hyperextension of plantar ligaments, or tarsal fractures, or both. Progressive, degenerative hyperextension of the joint is also seen in middle-aged or older, overweight dogs, especially of the collie or Shetland sheepdog breeds (1,4). Whether the cause is traumatic or degenerative, the patient presents with moderate to severe lameness and 20 to 30 degrees of plantigrade angulation at the tarsal joint (Figure 1). In chronic cases with significant plantigrade angulation, the digital flexor tendons are placed under continual tension, which can result in hyperflexion of the digits, or a claw-like appearance in the standing animal (4). Radiographs taken when the joint is being stressed can be helpful in identifying the site of the instability. Less commonly, trauma may result in damage to dorsal supporting ligaments in the tarsal and tarsometatarsal regions. These injuries produce chronic lameness with little or no gross deformity in the standing animal. Diagnosis depends on thorough palpation. Since weight-bearing produces compression of dorsal joint surfaces, rest, external coaptation, or both will frequently bring resolution (1). Tarsometatarsal hyperextension or luxation can also be traumatic or degenerative in origin and results from damage to plantar fibrocartilage, plantar supporting ligaments, collateral ligaments, or all 3. Again, collies and Shetland sheepdogs are commonly reported with degenerative hyperextensions (1,4). Any treatment except arthrodesis is doomed to failure. Indeed, it has been suggested that postoperative complication rates are significantly higher in patients that have received casts or splints before surgery (5).

Figure 1. A 45 kg Shepherd-cross dog with lameness and a plantigrade stance in the left hind limb. This stressed lateral view of the tarsal joint shows calcaneoquartal laxity due to plantar ligament rupture. Note the periarticular osteophytes associated with ligament rupture and chronic instability.

Several methods have been suggested for achieving arthrodesis of the proximal intertarsal or tarsometatarsal joints. To achieve success, regardless of the method, the principles of arthrodesis must be adhered to; that is, complete removal of articular cartilage; cancellous bone grafting of the joint space; and rigid internal fixation (preferably under compression), supported by external coaptation, until there is radiographic evidence of fusion (2). Articular cartilage can be debrided with power tools, but patient use of bone rasps and curettes will also suffice. Autogenous cancellous graft is readily attainable from the proximal extremity of the ipsilateral tibia, which must be included in the surgical preparation and draping.

Arthrodesis has been accomplished by using external fixators and intramedullary pins, with or without cross pins and with or without tension band wires (1,2,3,4,5). Bone plate fixation probably represents the best combination of rigid fixation, fewest complications, and technical ease of application. Lateral application of a dynamic compression plate that is attached to the calcaneus, 4th tarsal bone, and 5th metatarsal bone is most common (1,2,4,5,6) (Figure 2). In cases of calcaneoquartal injury, the screws on either side of the calcaneoquartal joint can be inserted off center, so as to produce compression and thus facilitating fusion. In tarsometatarsal hyperextension or luxation (Figure 3), inserting the screws on either side of the joint so as to produce compression may result in a valgus deformity and should be avoided (4). Tarsometatarsal instability can be treated with arthrodesis of the distal intertarsal and tarsometatarsal joints only by placing plate screws into the 4th tarsal and metatarsal bones. Medially, these screws are also seated in the central tarsal bone, the numbered tarsal bones, and the 4th metatarsal bone. There is evidence that fusing only the distal intertarsal and tarsometatarsal joints may predispose to degenerative joint disease of the proximal intertarsal joint (1). Fusion of the proximal joint level by extension of the plate onto the calcaneus may be prudent, especially if there is any question as to the integrity of the proximal intertarsal joint.

Figure 2. A postoperative view of the dog shown in Figure 1. Arthrodesis has been performed with the application of a 6-hole, 3.5-mm dynamic compression plate applied to the lateral side of the tarsus.

Figure 3. A 10-year-old shih-tzu with tarsometatarsal luxation.

Plate size for calcaneoquartal and tarsometatarsal arthrodesis will vary with patient size, but, in general, dogs weighing over 35 kg will require a 3.5-mm plate, and those weighing 10 to 35 kg, a 2.7-mm plate. Veterinary cuttable plate (VCP) (Veterinary Cuttable Plate; Synthes, Mississauga, Ontario), cut to an appropriate length, works well in cats and small dogs (Figure 4). The close spacing of screw holes in the VCP permits adequate numbers of screws in a small space of tarsus. If the arthrodesis plate is extended proximally, 2, and preferably 3, screws are placed in the calcaneus, 2 are placed in the numbered and central tarsal bones, and 2, or preferably 3, are anchored in 4th and 5th metatarsal bones (1,2).

Figure 4. Postoperative view of the dog shown in Figure 3. Arthrodesis has been performed with the application of a 5-hole length of 2.0/2.7 veterinary cuttable plate.

Recently, the use of the carpal arthrodesis plate (CAP) (Jorgenson, Fort Collins, Colorado, USA) via a plantar approach has been described for intertarsal and tarsometatarsal arthrodesis (7). The CAP was developed for pancarpal arthrodesis by application to the dorsal surface of the distal radius, radial carpal bone, and 3rd metacarpal bone. One of its most attractive features is that it is designed to hold larger 3.5-mm screws in the radius and 2.7-mm screws in the smaller metacarpal bone (8). In tarsal applications, following plantar dissection, the CAP is applied under compression to the calcaneus, 4th tarsal, and 4th metatarsal bones. Larger 3.5-mm screws can be placed in the calcaneus, while 2.7-mm screws are used in the thinner metatarsal bone. Plantar placement of the CAP is also more biomechanically sound. The plantar surface of the tarsus is the “tension side,” while the dorsal surface is the “compression side.” This means that under weight-bearing loads, the dorsal joint surfaces compress while the plantar surfaces are under tension. The application of a dynamic compression bone plate allows the tension forces on the plantar surface caused by weight-bearing to be converted to compressive forces at the arthrodesis sites. A plate on the dorsal surface would be subject to cyclic bending forces and would be prone to failure without significant additional support.

Postoperative external support with a cast or splint is essential in most surgical procedures, except for the plantar CAP arthrodesis and others that make use of the tension band principle, to protect the implants from excessive loading while fusion progresses. Even in the case of the CAP procedure, the author provided light external support. External support should be maintained until there is radiographic evidence of significant joint fusion, usually in 4 to 8 wk (1,2,3,4,5). Implant removal is usually not necessary, but it may be needed in cases of implant migration or breakage, osteomyelitis, or persistent lameness after fusion is complete.