Total Hip Replacement (THR)

Pathophysiology of Coxofemoral OA

Hip dysplasia is characterized by abnormal development and varying degrees of coxofemoral joint laxity. This laxity leads to subluxation, resulting in abnormal wear on the articular cartilage, microfractures of the subchondral bone, and eventual severe osteoarthritis (OA). While the onset of clinical signs does not always occur bilaterally simultaneously, dysplasia is almost universally a bilateral condition.

Clinical presentation ranges from subtle gait alterations (e.g., "bunny hopping" at a run) and reluctance to climb stairs, to severe hindlimb lameness and muscle atrophy. Total Hip Replacement (THR) remains the definitive surgical intervention, designed to restore the normal anatomical centre of rotation and provide lifelong, pain-free mobility.

Fixation Systems: CFX, BFX & KYON

Since the first canine THR in 1957, implant technology has advanced exponentially. BioMedtrix introduced the CFX® (Cemented Fixation) system in 1990 and the BFX® (Biologic Fixation) in 2003. To date, over 37,550 procedures have been performed using these implants on patients ranging from 1.5 kg to 80 kg, including felines. Modern veterinary surgeons typically select between cemented and cementless systems based on patient morphology.

Pre-operative Templating & Sizing

Successful THR relies heavily on meticulous pre-operative planning. Using calibrated digital radiographs and CAD templating software, the surgeon accurately determines the appropriate size for the femoral stem, femoral head, and acetabular cup.

During templating, the surgeon assesses the Canal Flare Index (CFI) to decide between cemented or cementless stems, ensuring the implant achieves an adequate cortical index (fill). For the acetabulum, the goal is to select a cup size that maximizes coverage while preserving the medial subchondral bone to prevent intrapelvic protrusion.

The Hip vs. Stifle (CCL) Dilemma

A classic diagnostic trap in veterinary orthopaedics is the large-breed dog presenting with hindlimb lameness and dramatic radiographic evidence of hip dysplasia. Studies indicate that up to 30% of dogs referred for hip dysplasia actually suffer from cranial cruciate ligament (CCL) rupture as the primary cause of their acute lameness.

Biomechanical Superiority over FHO

For dogs with severe hip dysplasia, the primary surgical alternative to a THR is a Femoral Head Ostectomy (FHO). An FHO is a salvage procedure that removes the femoral head and neck, relying on scar tissue to form a pseudoarthrosis (false joint). While an FHO eliminates bone-on-bone contact, it severely alters the biomechanics of the hip joint, resulting in limb shortening and permanent gait alterations.

Clinical Outcomes: Pain relief following THR is significantly more predictable than after FHO. Objective kinematic research using force plate analysis demonstrates that dogs return to normal Peak Vertical Force (PVF) within three to six months following THR. FHO outcomes are less predictable, particularly in dogs over 20 kg, and patients often require long-term NSAID therapy. THR eliminates the need for chronic drug therapy, mitigating the risks of long-term gastrointestinal or renal complications.

Complication Profiles & Recovery

The success rate for THR is remarkably high (routinely cited at >90-95%), and the implants are designed to last the patient's lifetime. However, it is a technically demanding procedure with a known complication profile. Total complication rates hover around 5-10%.

• Luxation: The most common early complication, usually occurring in a craniodorsal direction due to inadequate muscle mass, poor implant version/angle, or failure to restrict post-operative activity.
• Femoral Fracture: Fissures can occur during broaching of the femur or post-operatively (subsidence) before biologic integration is complete.
• Infection: A devastating complication requiring implant removal. Meticulous sterile technique and prophylactic antibiotics are critical.
• Neuropraxia: Transient sciatic nerve injury can occur due to retraction during surgery but typically resolves within several weeks.

Patients are typically hospitalized for 1-3 days. While they often begin weight-bearing within 24 hours, strict leash restriction and supervised rehabilitation are mandatory for 6-8 weeks to allow for soft tissue healing and, in the case of BFX systems, initial osteointegration.