Traumatic injuries do not require an osteotomy, but deforming arthritic injuries may require a corrective osteotomy for realignment of the limb. For simple degenerative arthritis without limb deformity an osteotomy is not necessary. If an osteotomy is needed, it is performed through the affected arthritic joint. For a partial carpal arthrodesis this is always the affected joint. For a pan-carpal arthrodesis the osteotomy is performed through the most deformed articulation.
The angulation of the osteotomy is estimated from the preoperative radiographs. The postoperative goal after the osteotomy is to have the limb in slight flexion with slight remaining varus deviation to assure that the tension surface of the reconstruction is in the location where the plates will be placed.
Therefore the plane of the osteotomy should diverge slightly from dorsal to palmar and should remove a wedge of bone smaller than the calculated amount as determined from the radiographs. If the osteotomy is done in the antebrachial carpal joint the surgeon must be aware that the accessory carpal bone is present in the posterior aspect of that joint and stop the osteotomy short of that bone.
Once the osteotomy is created the wedge of bone to be discarded is removed using the bone saw and rongeurs. The limb is then corrected into the new alignment and a radiograph taken if needed.
4. Surgical technique
The plates are normally located craniomedially and craniolaterally over the intermediate and radial carpal bones of the proximal row. Occasionally, the plates will be placed over the intermediate and ulnar carpal bones if marked realignment of the limb is necessary.
Application of the first plate
For an antebrachial carpal joint arthrodesis, the medial plate is attached to the radius first and extended distally onto the radial carpal bones.
Once the plate is attached proximally the tension device is attached to the distal end of the plate and used for compression across the osteotomy site. Normally only the first plate requires use of the tension device. Axial compression with the second implant can be achieved by using the dynamic compression holes. If no osteotomy is present and significant reduction of a deformity is not necessary the dynamic compression capabilities of the implant may be sufficient to achieve compression across the joint to be fused. The tension device should be placed distally in most instances because it is ill-advised to leave an open hole to act as a stress concentrator in the weakened distal radius of an osteopenic limb as it predisposes to postoperative fracture of the radius. Therefore, the tension device can be placed across the middle carpal joint and attached to the proximal McIII and then removed leaving the middle carpal joint unaltered after compression is achieved.
Once sufficient compression is achieved using the tension device in the first plate, the remaining screws are inserted into the radial carpal bones.
Application of the second plate
The tension device is removed and the second plate is attached using appropriate dynamic compression or locking plate principles.
Radiographs are taken after all screws are inserted to assure that all screws are in the proper location and the joint is been well reduced.
Antibiotic containing implants (PMMA beads or collagen sponges) are placed alongside both plates and the exposure closed using the annular ligament of the carpus, the subcutaneous tissue and the skin sequentially. Suction drainage may be used in cases with considerable intraoperative manipulation or in cases of trauma induced carpal injury.
After closure a sterile dressing is placed on the wound and a cast is applied from the elbow to the distal aspect of the metacarpus just above the metacarpophalangeal joint. Allowing the metacarpophalangeal joint to maintain flexion and extension greatly increases the ability of the horse to move comfortably postoperatively. The cast is maintained in place for 30 days with a cast change at 2 weeks postoperatively. The cast change can be performed standing or under general anesthesia depending on patient compliance. After cast removal full limb bandages are maintained until swelling is eliminated. Stall rest with no free-choice exercise is maintained for 60 days. Follow up radiographs at 60 days will determine the next step in the reconvalescent period. Normally 30 days of gradually increasing exercise is recommended prior to resumption of free-choice exercise. Partial arthrodesis is for non-athletic purposes only.