Slocum
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Quality Veterinary Equipment,
Implants and Diagnostic Aids
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Course 2008
Sept 12-13th TPLO course now accepting registration. |
DARthroplasty By Barclay Slocum, DVM and Theresa Devine Slocum, MS Introduction The treatment of hip dysplasia must be adapted to the stage of degeneration of the specific hip. Acetabular hip dysplasia is initiated when the developing hip is either too shallow or maloriented. As the femoral head begins to luxate from the acetabulum in the dysplastic hip, the femoral head can be captured by use of a pelvic osteotomy. This procedure reorients the acetabular segment of the pelvis in order to provide coverage to the femoral head and stability to the hip. There are three criteria which must be observed for the success of the Pelvic Osteotomy procedure. First, there must be sufficient cartilage on both the acetabulum and the femoral head. Second, a competent dorsal acetabular rim must be present. Third, the acetabulum must have a near normal radius of curvature and minimal osteophytes. Pelvic Osteotomy stabilizes the hip and reduces the stretching, and tearing of the joint capsule. The wear to the dorsal acetabular rim is stopped and thus the pain of capsular trauma is relieved. When the disease process has progressed beyond the criteria mentioned above, the Pelvic Osteotomy technique is not a viable surgical option. A Total Hip Replacement is indicated when there is bone on bone contact between the femoral head and the acetabulum creating pain and capsular stretching. The acetabulum is usually filled with osteophytes, which destroy the possibility of hip reduction. The teres ligament is torn as the subluxated femoral head rides dorsally providing contact of the femoral attachment of the teres ligament against the dorsal acetabular rim. As the teres ligament is destroyed, the femoral head migrates dorsally stretching the joint capsule, which provides more capsular pain. Since the femoral shaft in the young dog is flexible and the femoral prosthesis and cement are rigid, there is a higher incidence of Total Hip prosthesis loosening in the young patient than in the older patient. The expectation of longevity of the cemented total hip is five years prior to loosening in the young patient. Whereas the Total Hip can be expected to last the remainder of the patient’s life if implanted after the first five years of age. Therefore, the cemented total hip is best used after five years of age. This leaves a gap in the treatment algorithm for hip dysplasia between the Pelvic Osteotomy and the Total Hip Replacement. This void is filled by the DARthroplasty technique. The DARthroplasty accepts the hip in the luxated position and treats capsular pain by bone graft dorsal to the joint capsule. This augmented dorsal acetabular rim supports the joint capsule and thus prevents pain associated with capsular stretching. The DARthroplasty allows the patient to actively utilize the newly formed dorsal acetabular rim graft to return the hip to function. The presence of the DARthroplasty does not preclude future Total Hip Replacement, although that circumstance has rarely occurred. Standard DARthroplasty The standard DARthroplasty is indicated for hip dysplasia when the hip is luxated from the acetabulum, the femoral head is supported by the joint capsule and the teres ligament has been stretched to its limit. This allows the femoral head to contact the dorsal acetabular rim just dorsal to the fovea capitis. When the head is in the luxated position, the joint capsule and teres ligament will be undergoing the stress and pain of stretching. The inflammation within the joint under these circumstances will stimulate pannus in the depth of the acetabulum which results in the formation of osteophytes beginning at the margin of the acetabular fossa. Also trauma of the femoral head on the dorsal acetabular rim will begin to break or deform the dorsal acetabular rim. With either acetabular filling or breakdown of the acetabular rim, congruity of the hip or its stability becomes impaired, and use of a Pelvic Osteotomy is contraindicated because the radius of curvature of the acetabulum no longer matches the radius of curvature of the femoral head. Under these circumstances, a standard DARthroplasty can be performed, and the augmentation of the dorsal acetabular rim by bone taken from the wing of the ilium provides stability to this luxated hip. The bone graft lies outside of the joint capsule. This bone graft supports the capsule and the femoral head bears weight on the supported capsule. The femoral head usually maintains a position of capsular support as the bone graft matures and fuses to the pelvis. On occasion, the femoral head will migrate back into the acetabulum which gives acetabular support to the femoral head. Under this circumstance, the dorsal rim is excessive and creates a greater limitation to abduction than anticipated. Since patients experience a dramatic relief of capsular pain, and return of function, no owner has desired removal of the excessive bone. If the bone graft is insufficient to contain the femoral head, additional stretching will occur and joint instability is found on examination. Major DARthroplasty As the dysplastic process becomes severe, the femoral head migrates laterally and dorsally to allow the dorsal rim to bear direct weight on the teres ligament at its attachment on the femoral head. This causes destruction of the teres ligament and with that disruption of the ligament, the femoral head is allowed to migrate dorsally. This allows only the joint capsule to restrain further dorsal migration. These hips are painful, inflamed and have excessive synovial fluid. The patients often have "boxy hips" due to dorsolateral migration of the greater trochanter, and a base narrow stance due to the dorsally luxated position of the femoral head. Dysplastic hips of this nature often have filling of the acetabulum and yet the cartilage of the femoral head and acetabulum is intact despite the extremely rapid progression of the dysplastic process. In these young patients, the femoral neck is usually free of osteophytes and its spherical nature provides a good articular surface for articulating with the dorsal support. A bone graft placed dorsal to the acetabulum must extend as far dorsally as necessary to bridge over the severely stretched capsule in order to provide dorsal support of the femoral head. Although these patents initially walk with a base narrow gait due to contracture of the joint capsule, as the ventral joint capsule stretches and the bone graft matures, base normal is achieved. Many such patients have returned to functional normality even though the newly acquired acetabulum is dorsal and lateral to the original. None of these cases have had a ventral or medial migration of the femoral head into the acetabulum. Minor DARthroplasty The development of the acetabulum may be normal in its orientation but shallow in its depth. Under these circumstances reorientation of the acetabulum through Pelvic Osteotomy will lead to over rotation of the acetabular segment without appropriate acetabular support. Rather than use a Pelvic Osteotomy, a minor DARthroplasty can be used to extend the dorsal acetabular rim laterally. As the bone graft matures, the femoral head cannot go laterally far enough to round the lateral margin of the DARthroplasty. Consequently, the femoral head simply returns to the acetabulum without tearing or damage to the joint capsule. The hip structures are usually in excellent condition and the femoral head returns to the acetabulum with normal cartilage bearing weight on normal cartilage. The minor DARthroplasty has provided stability to a marginally stable hip and the remainder of the hip function is normal. Discussion The dorsal approach to the hip joint severs no muscles and the only structure that is disturbed in its origin is the deep gluteal muscle. This muscle is elevated from the dorsal and cranial acetabulum and joint capsule. This aspect of the approach probably denervates the joint capsule which provides relief of capsular pain following surgery. Elevation of the deep gluteal muscle also causes some patients to have a scissors gait in the immediate postoperative period but a base normal gait follows as healing progresses. Care must be taken in placing the ilial bone graft over the dorsal acetabulum. If the graft is excessive, abduction will be less than 35 degrees. If abduction is maintained at 35 degrees or greater, the dog will have normal function. If an excessive amount of bone graft is placed dorsally or caudally beneath the gemelli muscles, it is possible to stretch the sciatic nerve causing a proprioceptive deficit such as knuckling of the digits. The most interesting characteristic of the DARthroplasty procedure is the ability of the hip to become stable with minimal discomfort to the patient. DARthroplasty surgeries are most often performed bilaterally. As the bone graft matures into a new extended dorsal acetabular rim, the patients will often acquire a puppy like attitude which is often maintained for years. The bone graft of the DARthroplasty initially has a notch where the joint capsule is reduced to a layer of tissue that is equal or greater in thickness to the articular cartilage. The success of this procedure in restoring dogs to pain free function has far exceeded our expectations.
Related Articles:
DARthroplasty Cases: Standard DARthroplasty, Major DARthroplasty
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