Clinic for Small Animal Surgery, Department of Small Animals, Vetsuisse-Faculty, University of Zurich, Zurich, Switzerland
Hip dysplasia (HD) is a common multifactorial, developmental disease of the hip joint in dogs. The disease has variable incidences depending on the breed and is a common cause for presentation of a dog with lameness. It is important for the veterinarian to have an understanding of the etiology, pathophysiology, biomechanics, diagnostic techniques, and treatment options associated with HD. The veterinarian must be able to explain the rationale behind diagnostic and breeding programs, as well as, advise the client concerning treatment.
Many causes of HD have been proposed and investigated. It is now known that there is a genetic component to the disease. Dogs are predisposed to development of HD through a polygenic mode of inheritance. Affected dogs are predisposed to developing a biomechanical imbalance between muscle mass and skeletal stresses on the hip joint. However, other factors including nutrition, hormonal influences, cell and matrix abnormalities, and environmental factors may be equally important in the development of HD. The dogs that are genetically predisposed to HD develop laxity in the hip as a result of a biomechanical imbalance between the forces on the joint and the associated muscles mass. The laxity leads to incongruity in the joint, which eventually results in osteoarthritis. It is important to remember that the hip joint is normal at birth. During the first 60 days of life, the muscles and nerves are functionally limited. If the stresses of weight-bearing and activity exceed the elastic limit of these tissues, the joint laxity results in irreversible incongruity during development. Stabilization of the hip is afforded by muscles, joint capsule, the round ligament, and the homeostatic hydrostatic pressure in the joint. When joint congruity is lost by insufficiency of these structures, changes in joint shape can only be accomplished by the production or resorption of bone.
Diagnosis of HD is based on signalment history, complete general physical examination, complete orthopedic examination, neurologic examination, and radiography. Traditionally, dogs presenting with signs of HD have been divided into 2 groups. Dogs of less than 12 months of age often present with a sudden onset of clinical signs including decreased activity, hip pain, gait alteration, and/or lameness attributed to joint effusion, ligament strain, synovitis, acetabular microfractures, and/or articular cartilage loss. Older dogs most often present with chronic and insidious signs attributed to osteoarthritis. Clinical signs in these dogs include lameness (especially after exercise), joint crepitus, restricted range of mot ion (especially extension), and muscle atrophy. It is important to remember that each dog must be evaluated on an individual basis as many clinical signs can vary and overlap between groups.
In the young dog early diagnosis is crucial. Both palpation technique and specific radiographic techniques can be useful. Palpation techniques are helpful in establishing a diagnosis and deciding on a treatment plan. The Ortolani sign is a very useful technique for determining hip joint laxity and gaining information for potential treatment options. The technique can be performed on awake or sedated patients, but best results are obtained in anesthetized dogs. We strongly recommend that all patients of susceptible breeds be palpated for the Ortolani sign when anesthetized for neutering.
Specific radiographic techniques have been developed to measure laxity in young dogs.
The University of Pennsylvania Hip Improvement Program (PennHIP) radiographic technique is a stress radiographic method intended to provide a quantitative means of determining hip laxity before the dog is 24 months of age. This method is intended to provide early and optimal predictive value by correlating joint laxity with subsequent incidence of CHO. A compression and a distraction view are obtained using the PennHIP compressiondistraction device with the patient under deep sedation or general anesthesia. The distance between the center of the acetabulum and the center of the femoral head is measured on both views using templates or gauges. This distance is divided by the radius of the femoral head; a numerical value between 0 and 1, the distraction index, is determined. The distraction index quantitates passive joint laxity. In some breeds, an increase in the distraction index has been correlated with an increased incidence of OA associated with CHO. The dorsal acetabular rim (DAR) radiographic view is another technique that was developed to evaluate the DAR using standard radiographic equipment and technique. The DAR view was designed to address the failure of standard ventrodorsal and latera l radiographic techniques to isolate the weight-bearing portion of the acetabulum. The value of the DAR view is in evaluating the DAR for damage and secondary OA changes, correlating palpable joint laxity with observable radiographic findings, and displaying acetabular filling. This technique has been proposed to provide an objective, reproducible method for determining whether a hip is normal, dysplastic, or injured by trauma.
The treatment of the young dog with HD aims at preventing of DJD. The earliest treatment option for preventing DJD is pubic symphysiodesis. This technique is designed to alter the normal growth of the pelvis to allow acetabular rotation similar to what is accomplished via the TPO procedure. The pubic symphysis is iatrogenically damaged with electrosurgery or staples early in the dog’s development (3–5 months of age) so that endochondral ossification stops. This focal disturbance of growth result in relatively less growth of the ventral pelvis, which results in bilateral acetabular rotation and increased femoral head coverage. Because the technique needs to be performed in very young puppies, early diagnosis is critical. This technique has the advantages of being minimally invasive, resulting in bilateral acetabular rotation, and can be performed in conjunction with a neutering procedure. Early results of this technique in clinical dogs have been very promising.
Triple or double pelvic osteotomy is another surgical procedure designed for the young dog. Ideally, correction takes place prior to skeletal maturity and before secondary changes occur. The goals of TPO are correction of femoral head subluxation and restoration of the hip’s weight-bearing surface area. The ideal candidate for TPO is a young dog (<10 months of age) with clinical signs of CHO, radiographic subluxation, and no secondary OA changes. The procedure involves performing osteotomies in the pubis, ischium, and ilium to allow axial rotation of the acetabulum providing increased dorsal acetabular coverage and weight-bearing surface area. The ilial osteotomy is then stabilized using a bone plate that maintains the desired degree of rotation. A recent study has shown that no advantage in femoral head coverage or weight bearing surface area is gained with higher degrees of rotation. Postoperative management involves exercise restriction until radiographically evident healing of the ilial osteotomy, followed by a gradual return to normal function. Complications associated with TPO include narrowing of the pelvic canal, constipation, urethral injury, overrotation of the acetabulum (resulting in limited femoral extension and abduction), implant failure, infection, sciatic nerve palsy, persistent incongruity, and failure to retard the progression of OA. The critical issue when choosing if to perform DPO or TPO is that the ideal candidates are dogs without symptoms, which poses a question on the ethical decision to perform an invasive procedure with risk of complications. On the other hand, when waiting for clinical signs to present, early signs of DJD may progress and the candidate may not be ideal.
The treatment of the chronic HD follows the principles of management of degenerative joint disease (DJD). Medical management is based on client education and comprehensive patient management. In order to efficiently and accurately diagnose, treat, and determine prognoses for DJD patients in practice, it is important to understand the basics of this condition. Although historically described as a “non-inflammatory” disease, it is now accepted that DJD is an inflammatory condition, but the inflammation is not mediated by neutrophils as in other types of arthritis. Many, many cells and cytokines are involved in the vicious cycle of DJD and it is certainly not yet comprehensively understood. For this reason the medical treatment of DJD aims at decreasing the inflammation in/around the joint. Strategies to treat DJD associated with HD include weight loss, low impact exercise (underwater therapy and swimming are excellent), anti-inflammatory drugs and chondroprotective agents.
When medical management fails, surgical treatment for HD should be selected. THR is the best treatment option for dogs that are clinically affected with OA resulting from CHO (our opinion). Contraindications include infection, neurologic disease that affects the hindlimbs, concurrent orthopedic problems, and some systemic diseases. THR can be done bilaterally with at least 2–3 months between surgeries. Unilateral THR, however, reportedly results in acceptable function in up to 80% of dogs with bilateral CHO. Postoperative management is vital to successful outcome. For the first week after surgery, activity is restricted to leash walking with support of the hindlimbs. Activity restriction should continue for the first month, with a gradual return to function 10–12 weeks after surgery. THR is reported to have a 91–95.2% success rate. Complication rates vary widely and decrease with increased surgical experience. Complications include dislocation, osteomyelitis, aseptic component loosening, femoral fractures, and sciatic neuropraxia. Infection is the most severe complication.