Surgery STAT: Micro total hip replacement for cats and small dogs

Editor's Note: SurgerySTAT is a collaborative column between the American College of Veterinary Surgeons (ACVS) and DVM Newsmagazine.

Hip dysplasia, hip luxation, femoral head and neck fractures and avascular necrosis of the femoral head are painful conditions that lead to osteoarthritis (OA) in dogs and cats. OA medical management includes analgesics to relieve pain, oral chondroprotective agents, rehabilitation therapy, viscosupplementation injections and dietary restrictions to maintain a lean body condition. Some dogs and cats, just like humans, may reach a point where medical management of pain is insufficient. Surgical intervention should be considered when clinical signs become refractory to medical management.

The contemporary surgical option to treat coxofemoral pathology in smaller dogs and cats is femoral head ostectomy (FHO). Numerous published reports about the clinical outcomes of FHO have provided subjective results, but objective data such as those generated from computerized force plate gait analysis evaluating long-term effects on large numbers of patients is sparse. The available objective patient data after FHO reveals a high percentage of suboptimal results. After FHO, the hindlimb is shortened to a variable degree, biomechanical function is severely altered, pain relief is unpredictable, muscle atrophy with weakness is a common long-term finding and postoperative rehabilitation is prolonged. On the contrary, published reports of objective measures taken following total hip replacement (THR) in large dogs consistently document a return to normal function.

Photo 1: A Micro THR prosthesis.

The goal of THR is a pain-free joint that mimics normal biomechanics with excellent long-term function. THR is a common procedure used to treat OA and other hip arthropathies in large dogs, and it should be considered in smaller patients. Micro total hip replacement (Micro THR) has been available for cats and small dogs since April 2005, with implants and instrumentation designed specifically for small patients (Photo 1).

Photo 2: Bilateral coxofemoral subluxation in a Lhasa Apso treated with staged bilateral Micro THR surgery procedures one month apart.

The smallest dog to receive a Micro THR to date is a 2.45-kg Maltese with avascular necrosis of the femoral head. The weight range for the Micro THR procedure is about 2.5 to 12 kg. The patient's acetabular cranial to caudal inside dimension must be ≥ 11 mm, and the inside diameter of the femoral medullary canal must be ≥ 3.5 mm to accept the prosthesis. Staged bilateral Micro THR surgery in dogs (Photo 2) and Micro THR in cats (Photo 3; also see the "Surgical Success" link below to view related videos) have been reported.

Photo 3: A ventrodorsal radiograph of a Micro THR in a cat (also see the "Surgical Success" link below to view videos of this same patient).

The basic principles for implanting the Micro THR are similar to those for the standard THR procedure. The prosthesis profile and design are smaller versions of larger sizes. The only unique thing about the procedure is that the patient, the prosthesis and the instrumentation are smaller. The complication rate is low, with prosthesis luxation being most common. Other complications, including those caused by technical errors, can be resolved by implant revision or by explantation resulting in an FHO.

More than 120 dog breeds weigh less than 12 kg, and numerous mixed-breed dogs also fall into this weight bracket. Hip pathology requiring surgical intervention to resolve pain or dysfunction can occur in any breed of dog or cat, so the indications for Micro THR are numerous.

Micro THR should be offered as a treatment option for small companion animals with hip arthropathy when the long-term prognosis for a fully functional, pain-free recovery is better than with other treatments. FHO will continue to be an alternative treatment choice, especially when client constraints, including financial ones, limit treatment to continued medical management or to this salvage procedure. However, clients should be advised of the advantages and disadvantages of both a THR and an FHO.

Dr. Liska is a founder of Gulf Coast Veterinary Surgery and Gulf Coast Veterinary Specialists in Houston. He became board-certified by the American College of Veterinary Surgeons in 1980.

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