Similar limb lengths needs to be maintained. The osteotomy must allow correction of the deformity but preserve the ability to achieve stable fixation. Therefore a multiple plane step or Z-shaped osteotomy is used in the horse rather than a single plane opening or closing osteotomy.
Frontal or sagittal plane step osteotomy
The long axis of the osteotomy can be oriented in either the sagittal or the frontal plane. The frontal plane step osteotomy (left) is technically easier to perform and therefore is used when ever possible. The sagittal plane step osteotomy (right) is more versatile for correcting compound or dual deformities and therefore is used with these more complicated situations.
Preoperative planning is very important when performing an osteotomy for deformity correction. The frontal plane osteotomy is performed with the mid-portion of the osteotomy (b) in the frontal plane midway between the dorsal and palmar/plantar cortex centered over the deformity. The proximal plane of the osteotomy (a) is performed parallel to the proximal joint surface of McIII/MtIII. The distal plane of the osteotomy (c) is performed parallel to the distal joint surface of McIII/MtIII.
Performing the osteotomy
Intraoperatively osteotomy planes a and c will be performed sequentially and followed by osteotomy plane b to connect the first two components of the osteotomy. Osteotomy planes a and c are both sawn from the surface of the bone 50% of the distance from dorsal to palmar/plantar in osteotomy a and 50% of the distance from palmar/plantar to dorsal in osteotomy c. Once these cuts are completed, osteotomy plane b is sawn in the frontal plane mid-way from dorsal to palmar/plantar in the bone to connect osteotomy planes a and c.
Dynamic condylar screw plates or locking plates are advantageous in this fixation, because of their increased strength. In both fixations the implants are used to create both axial and interfragmentary compression. In lighter individuals a single plate may be adequate, in larger individuals 2 plates placed at 90 degrees are stronger.
In the frontal plane osteotomy, the first plate is applied to the dorsal aspect of the bone. The plate is attached to the proximal fragment first. If a DCS plate is used, the barrel is placed in the proximal metaphysis. The long axis of the plate is used to guide axial alignment of the osteotomy fragments. Once the proximal plate is attached, the screws are placed in the distal osteotomy fragment using the dynamic compression principle to create axial compression.
Cortex screws are subsequently placed in lag fashion across the osteotomy to create interfragmentary compression.
If necessary the sharp edges of the osteotomy can be shaped with the help of the oscillating saw after fixation to protect the soft tissues.
4. Sagittal plane osteotomy
Preoperative planning is very important when performing an osteotomy for deformity correction. The sagittal plane osteotomy is performed with the mid-portion of the osteotomy (b) in the sagittal plane midway between the medial and lateral cortex centered over the deformity. The proximal limb of the osteotomy (a) can be oriented either medially or laterally, the distal limb (c) is then oriented towards the opposite cortex. The connecting limb of the osteotomy is made in the sagittal plane.
Performing the osteotomy
Intraoperatively osteotomy planes a and c will be performed sequentially followed by osteotomy plane b to connect the first two components of the osteotomy. Osteotomy planes a and c are both sawn from the surface of the bone 50% of the distance from medial or lateral to the mid-portion of the bone. Osteotomy plane a is created parallel to the proximal joint surface, osteotomy plane b is created parallel to the distal joint surface of McIII/MtIII. Once these cuts are completed, osteotomy plane b is sawn in the sagittal plane using two osteotomy planes, one perpendicular to osteotomy plane a (x) and one perpendicular to osteotomy plane c (y).
The saw cuts x and y in the sagittal plane create a wedge of bone from the dorsal cortex and the palmar/plantar cortex that are removed to correct the angulation.
Correction of rotation
If correction of rotation is required in addition to angulation, a slightly larger wedge is removed from the dorsal cortex to rotate the position of the distal limb.
In this example, the rotation correction is accomplished by increasing the size of the dorsal wedge.
Intraoperatively the correction is visualized by placing 2 mm marker drill bits parallel with the proximal and distal joint surfaces and perpendicular to the long axis of the limb at the site of drill bit placement. The correction is then achieved with the osteotomy until the drill bits are parallel in all planes. At this point the fixation is placed perpendicular to the sagittal osteotomy and interfragmentary and axial compression used to create stability.
Application of the first plate
A temporary fixation with a cortex screw applied in lag fashion allows taking radiographs to determine alignment. Once the alignment is correct the largest plate is placed on the lateral aspect of the bone. The plate is used to create axial compression using the dynamic compression principles and interfragmentary compression using cortex screws in lag fashion across the osteotomy plane.
Application of the second plate
In sagittal osteotomies double-plate fixation is most often used. The second plate is applied to the dorsal aspect of McIII/MtIII at 90 degrees to the lateral broad DCP for greater rotational stability.
In this instance, a 10-hole narrow DCP has been used.
Antibiotic containing implants (PMMA beads or collagen sponges) are placed along the plates, suction drainage can be used if needed. The longitudinal incision in the extensor tendon is reapposed and followed by closure of the subcutaneous tissues and skin.
Cast immobilization is usually not used, but can be applied if needed. Sterile bandages are placed on the limb. Stall rest is maintained for 6-8 weeks. Radiographs are taken at that time to determine the next step in the reconvalescent period. Implant removal is necessary for high-level athletic activity such as racing, but not for less strenuous uses.