The hip is a socket and ball joint allowing a large range of motion. During linear locomotion, flexion and extension are most prominent but abduction, adduction and rotation are important during turning, braking, acceleration, jumping, and landing. The most common hip problem in dogs and cats is hip dysplasia, which is characterized by increased laxity of the joint during active growth.¹ In dogs, hip dysplasia clearly is a clinical entity, but in cats this problem is most often subclinical and an incidental finding when performing radiographic imaging of the pelvis. In both dogs and cats, hip dysplasia is a dynamic problem which makes it difficult to assess clinically. Laxity of the hip joint can be assessed clinically using manipulation of the joint including the Ortolani test. Goal of these techniques is to evaluate subluxation and reduction of the femoral head in comparison with the acetabulum. Cooperation of the animal and/or sedation may be necessary to properly assess laxity of the hip joint. Disadvantage is that these clinical assessments do not represent the function of the hip joint under physical loading. Over-interpretation of laxity in young dogs may be the result. Another means of evaluating the presence of hip dysplasia is by radiographic imaging. The imaging techniques can be subdivided as static and dynamic protocols.

The most common static radiographic imaging view is the extended hip as used by the FCI for hip dysplasia screening in breeding dogs. In this view, congruity of the hip joint, acetabular coverage of the femoral head, Norberg angle, and secondary osteophyte formation can be assessed.² As stated, this is a static view which does not represent the standing angle of the hip joint. The PennHip method is more dynamic in nature, as it tries to assess the stability of the femoral head in the acetabulum resulting in a laxity index.^3,4 Nevertheless, the PennHip method does not represent functional loading of the hip joint.

Laxity in itself does not lead to hip dysplasia in all dogs. Laxity of the hip joint during normal functional loading will lead to cartilage damage, clinical lameness, and will start progressive osteoarthritis. Treatment of hip dysplasia in young animals is aimed at restoring stability and preventing progressive joint damage. If laxity of the joint capsule is the major symptom, joint damage is still limited, and the acetabulum and femoral head are well developed; these dogs can be treated with a triple pelvic osteotomy (TPO), or a double pelvic osteotomy (DPO) in even younger dogs.^5,6 The goal of these surgical procedures is to rotate the dorsal acetabular rim laterally thus improving coverage of the femoral head. The procedures are not aimed initially at improving the laxity of the joint capsule. To accomplish rotation of the acetabulum, the ischium, pubis, and ilium are osteotomized thus mobilizing the acetabulum. A dedicated plate is used stabilize the ilial osteotomy while rotating the acetabular rim laterally, usually with 20 degrees. A DPO procedure uses the same pubic and ilial osteotomies but leaves the ischium intact, thus providing more stability for the mobilized acetabular segment. Again, a dedicated plate is used to stabilize the ilial osteotomy and rotate the dorsal acetabular rim laterally. It is crucial that the patient is young enough with sufficient elasticity in the ischial table and symphysis to accommodate the rotation of the acetabulum. Additional stability can be accomplished with both TPO and DPO procedures by using locking plate systems.

Although TPO and DPO are good options if the hip joint is still largely intact, prognosis will be poor with clear malformation and/or clear osteoarthritis. In these cases, treatment can be either conservatively or surgically with a total hip replacement. Conservative treatment including weight reduction, analgesics, and exercise therapy can be very rewarding. In case function cannot be restored and pain is a major problem, a total hip prosthesis can be indicated.

The total hip prosthesis systems can be divided in two groups in analogy with human hip prostheses.⁷ One group uses polymethyl methacrylate (PMMA) or bone cement to stabilize the prosthesis cup in the acetabulum and the shaft of the prosthesis in the femur. The PMMA provides a direct stabilization of the prosthesis component in the bone. The second group uses a press-fit of the prosthesis and relies on osteointegration or bone ingrowth into the surface of the prosthesis component for its final stability. This means that stability of the prosthesis system increases during the first 6 to 18 weeks after surgery. Both systems have their advantages and disadvantages and are comparable in final results and complications.

In human medicine, resurfacing procedures with replacement of only the acetabulum or femoral head have had some interest but proved to be far inferior to total hip replacement.

The major drawback of total hip replacement in dogs and cats is the cost of the implant. In view of this, a salvage procedure may be of interest. As pain from the hip joint usually is the major complaint, eradicating pain should be the main goal. A femoral head and neck resection can be effective in relieving hip pain. A disadvantage of the procedure is that the functional length of the femur is decreased, which must be compensated for by extending the hip, stifle and crurotarsal joint. This leads to an impaired locomotion of the affected hind limb but without pain. In cats and small dogs, it is very difficult to see this mechanical lameness, although it is present; but in larger dogs, it is clearly visible. The goal of a total femoral head and neck resection in treating chronic hip disease is not to restore normal locomotion but to alleviate pain. In dogs with Calve-Legg-Perthes disease or avascular necrosis of the femoral head complete femoral head and neck resection is the treatment of choice.⁸

In conclusion, early surgical treatment of dogs with clinical hip dysplasia can best be performed using a TPO or DPO technique. In older dogs with hip dysplasia, osteoarthritis, and chronic pain a total hip replacement is the most suitable to restore function. Femoral head and neck resection can be used as a salvage procedure.

References

1.  Lavrijsen IC, Heuven HC, Meij BP, Theyse LF, Nap RC, Leegwater PA, et al. Prevalence and co-occurrence of hip dysplasia and elbow dysplasia in Dutch pure-bred dogs. Prev Vet Med. 2014;114(2):114–122.

2.  Janssens L, De Ridder M, Verhoeven G, Gielen I, van Bree H. Comparing Norberg angle, linear femoral overlap and surface femoral overlap in radiographic assessment of the canine hip joint. J Small Anim Pract. 2014;55(3):135–138.

3.  Smith GK. PennHIP more than "complementary" to FCI hip screening method. J Small Anim Pract. 2009;50(2):105–106.

4.  Runge JJ, Kelly SP, Gregor TP, Kotwal S, Smith GK. Distraction index as a risk factor for osteoarthritis associated with hip dysplasia in four large dog breeds. J Small Anim Pract. 2010;51(5):264–269.

5.  Slocum B, Devine T. Pelvic osteotomy in the dog as treatment for hip dysplasia. Semin Vet Med Surg (Small Anim). 1987;2(2):107–116.

6.  Vezzoni A, Boiocchi S, Vezzoni L, Vanelli AB, Bronzo V. Double pelvic osteotomy for the treatment of hip dysplasia in young dogs. Vet Comp Orthop Traumatol. 2010;23(6):444–452.

7.  Bergh MS, Budsberg SC. A systematic review of the literature describing the efficacy of surgical treatments for canine hip dysplasia (1948-2012). Vet Surg. 2014; Epub May 19.

8.  Alpaslan AM, Aksoy MC, Yazici M. Interruption of the blood supply of femoral head: an experimental study on the pathogenesis of Legg-Calve-Perthes Disease. Arch Orthop Trauma Surg. 2007;127(6):485–491.