Canine Hip Dysplasia

Canine hip dysplasia (CHD) is a developmental orthopedic disease characterized by a malformed coxofemoral joint, leading to laxity, subluxation, and progressive degenerative joint disease (DJD). It is a polygenic, heritable condition influenced by environmental factors. The disease encompasses a spectrum from mild joint laxity to severe osteoarthritis, with clinical signs typically emerging during growth or later in life. Surgical management aims to alleviate pain, improve joint stability, and delay or prevent the progression of osteoarthritis. Key surgical interventions include juvenile pubic symphysiodesis (JPS), triple pelvic osteotomy (TPO), femoral head and neck excision (FHNE), and total hip replacement (THR). The choice of procedure depends on the patient's age, degree of laxity, presence of osteoarthritis, and surgeon expertise.

The primary etiology of CHD is multifactorial, involving genetic predisposition, rapid growth, and environmental influences. Genetic factors are paramount, with heritability estimates ranging from 0.2 to 0.6 in various breeds. Multiple genes contribute to the phenotype, affecting hip joint conformation, ligamentous laxity, and muscle mass. Environmental factors include excessive caloric intake, rapid weight gain, and over-exercising during the growth phase. Nutritional imbalances, particularly excess calcium and vitamin D, can exacerbate the condition. The underlying biomechanical trigger is the incongruity between the femoral head and acetabulum, leading to abnormal weight distribution, joint capsule stretching, and microtrauma. This results in synovitis, cartilage erosion, and eventual osteoarthritis. Iatrogenic factors, such as improper surgical techniques or trauma, can also induce secondary hip dysplasia, though rare.

CHD is most prevalent in large and giant breed dogs, with a higher incidence in breeds such as German Shepherds, Labrador Retrievers, Golden Retrievers, Rottweilers, and Bernese Mountain Dogs. It is less common in small breeds and cats, though it can occur. The disease affects both sexes, but some studies suggest a slight female predisposition. Age of onset varies: clinical signs may appear as early as 4-6 months in severe cases, while others may not show signs until later in life. The prevalence in high-risk breeds can exceed 50%, with radiographic evidence of hip laxity in up to 70% of some populations. Working dogs, such as police and military dogs, are particularly vulnerable due to high physical demands, which can accelerate the progression of osteoarthritis. Genetic screening programs, such as the Orthopedic Foundation for Animals (OFA) and PennHIP, have been implemented to reduce the incidence, but the disease remains a significant clinical problem.

The pathophysiology of CHD involves a cascade of biomechanical and inflammatory events. Initially, there is a mismatch between the femoral head and the shallow acetabulum, leading to joint laxity. This laxity allows the femoral head to subluxate, especially during weight-bearing, causing abnormal stress on the articular cartilage and joint capsule. The joint capsule becomes stretched and inflamed, leading to synovitis. Synovial fluid becomes less viscous and contains inflammatory mediators such as prostaglandins, cytokines, and matrix metalloproteinases. These mediators degrade cartilage proteoglycans and collagen, leading to cartilage fibrillation and erosion. Subchondral bone undergoes remodeling, with osteophyte formation at the joint margins. The joint capsule thickens and fibrosis develops, further restricting motion. Over time, the progressive osteoarthritis results in chronic pain, muscle atrophy, and decreased range of motion. In severe cases, the femoral head may completely luxate, causing acute lameness.

Intrinsic predisposing factors include genetic susceptibility, with certain breeds having a higher risk due to selective breeding for specific conformations. Conformational traits such as a steep acetabular slope, shallow acetabulum, and poor muscle mass around the hip increase the risk. Metabolic factors, including rapid growth rate and obesity, exacerbate the condition. Age is a significant factor, with puppies undergoing rapid growth being most vulnerable. Extrinsic factors include nutritional mismanagement, such as overfeeding and excessive calcium intake, which can accelerate growth and worsen joint laxity. Trauma to the hip joint can also precipitate or worsen the condition. Prior surgeries, particularly those that alter joint biomechanics, can predispose to secondary dysplasia. Excessive physical activity, especially high-impact exercises like jumping and running on hard surfaces, can increase the risk of developing clinical signs.

Clinical signs of CHD vary with the severity and age of the dog. In young dogs (4-12 months), signs include hind limb lameness, especially after exercise, a 'bunny-hopping' gait, difficulty rising, and reluctance to run or climb stairs. On physical examination, there may be pain on hip extension and abduction, and a positive Ortolani sign (palpable reduction of the subluxated hip). In older dogs, signs are more insidious, with progressive lameness, muscle atrophy of the hind limbs, and a swaying gait. Pain may be elicited on palpation of the hip joint, and crepitus may be felt. In severe cases, there may be a noticeable decrease in range of motion. Systemic signs are uncommon but may include behavioral changes due to chronic pain. Lameness grading scales, such as the Canine Orthopedic Index, are used to quantify the severity.

Differential diagnoses for CHD include: 1) Legg-Calvé-Perthes disease, which typically affects small breeds and presents with similar signs but is due to avascular necrosis of the femoral head; 2) Cranial cruciate ligament rupture, which causes hind limb lameness but is localized to the stifle; 3) Lumbosacral disease, such as cauda equina syndrome, which can cause hind limb weakness and pain; 4) Hip luxation, which is usually traumatic and presents with acute lameness and a characteristic stance; 5) Septic arthritis, which presents with acute severe lameness, fever, and joint effusion; 6) Osteochondritis dissecans of the femoral head, which is rare but can mimic CHD; 7) Fractures of the pelvis or femur, which are traumatic and have a distinct history; 8) Neoplasia of the hip region, such as osteosarcoma, which is more common in older large breeds and presents with progressive lameness and pain. Definitive diagnosis is based on imaging and clinical findings.

The diagnostic algorithm for CHD begins with a thorough history and physical examination, including orthopedic and neurological assessments. The Ortolani and Barlow tests are performed to assess joint laxity. Radiography is the cornerstone of diagnosis. Standard ventrodorsal hip-extended view is used for OFA evaluation, while the PennHIP method uses distraction radiography to measure the distraction index (DI). A DI > 0.3 indicates laxity. Additional views include the frog-leg lateral and compression views. In young dogs, radiographs may be normal despite clinical signs, so advanced imaging such as CT or MRI may be indicated to assess joint congruity and early degenerative changes. Arthroscopy can be used to directly visualize cartilage damage and confirm the diagnosis. In cases where surgery is planned, a complete blood count, biochemistry profile, and coagulation panel are performed to assess surgical risk.

Laboratory findings in CHD are generally nonspecific. Synovial fluid analysis may reveal decreased viscosity, poor mucin clot, and increased cell count (typically < 5000 cells/µL) with a predominance of mononuclear cells. Inflammatory biomarkers such as C-reactive protein (CRP) and serum amyloid A (SAA) may be elevated in cases with active synovitis. Hematology and biochemistry are usually within normal limits unless there is concurrent disease. Coagulation panel (PT, aPTT, TEG) is essential for surgical planning to rule out bleeding disorders. Blood gas analysis may be indicated in cases with systemic illness. Urinalysis is part of the routine preoperative workup.

Radiography is the primary imaging modality. The standard ventrodorsal hip-extended view is used to evaluate the Norberg angle (normal > 105°), which measures acetabular coverage. The distraction view (PennHIP) provides a distraction index (DI) that predicts the risk of developing osteoarthritis. The compression view assesses joint reduction. In young dogs, radiographs may be normal, but the DI can be measured. Ultrasonography can be used to assess soft tissue structures around the hip, but it is less commonly used. CT provides 3D reconstructions of the hip joint, allowing precise measurement of acetabular version and femoral anteversion, which is useful for surgical planning. MRI is excellent for evaluating cartilage and soft tissue, and can detect early cartilage damage. Arthroscopy is the gold standard for assessing cartilage lesions and can be used therapeutically. Fluoroscopy is used during surgery to guide implant placement.

Cytology of synovial fluid in CHD typically shows a mild inflammatory pattern with increased cellularity, predominantly mononuclear cells. Histopathology of the joint capsule reveals synovial hyperplasia, fibrosis, and infiltration of inflammatory cells. Cartilage biopsies show fibrillation, erosion, and loss of proteoglycans. In cases undergoing THR, the femoral head is submitted for histopathology to rule out neoplasia. In cases of FHNE, the excised tissue is examined to confirm the absence of infection or neoplasia. Special stains such as Safranin O and Toluidine blue are used to assess cartilage glycosaminoglycan content.

Treatment of CHD is tailored to the individual patient. Medical management includes weight control, exercise modification, and administration of NSAIDs (e.g., carprofen 2.2 mg/kg PO q12h), chondroprotectants (e.g., polysulfated glycosaminoglycan 5 mg/kg IM q7d for 8 weeks), and analgesics (e.g., tramadol 2-5 mg/kg PO q8h). Surgical options include: 1) Juvenile pubic symphysiodesis (JPS) for puppies 12-20 weeks old with mild laxity, which involves thermal ablation of the pubic symphysis to alter pelvic growth; 2) Triple pelvic osteotomy (TPO) for dogs < 10 months old with minimal osteoarthritis, which involves osteotomies of the ilium, ischium, and pubis to rotate the acetabulum; 3) Femoral head and neck excision (FHNE) for salvage in cases with severe osteoarthritis or when THR is not feasible, which involves excision of the femoral head and neck to create a false joint; 4) Total hip replacement (THR) for dogs with severe pain and osteoarthritis, which involves replacing the femoral head and acetabulum with prosthetic components. Postoperative care includes pain management, restricted activity, and physical therapy. Complications include infection, implant loosening, and sciatic nerve injury.

The prognosis for CHD varies with the severity and treatment. With medical management, many dogs can have a good quality of life, but progression of osteoarthritis is inevitable. JPS and TPO have good to excellent outcomes in appropriately selected patients, with 80-90% of dogs returning to normal function. FHNE provides pain relief in 80-90% of cases, but functional outcome is variable, especially in large dogs. THR has the best outcomes, with 90-95% of dogs achieving excellent function and long-term implant survival. Complications such as infection, implant loosening, and luxation occur in 5-10% of cases. Negative prognostic indicators include severe osteoarthritis, obesity, and concurrent orthopedic disease.

Postoperative follow-up is crucial. For JPS, radiographs are taken at 6 months to assess pelvic conformation. For TPO, radiographs are taken at 4, 8, and 12 weeks to assess bone healing. For FHNE, radiographs are taken at 4 weeks to assess the formation of the false joint. For THR, radiographs are taken at 4, 8, 12 weeks, and then annually to monitor implant stability. Activity is restricted for 6-8 weeks postoperatively, with gradual return to normal over 3-6 months. Physical therapy, including passive range of motion exercises and swimming, is initiated early. Long-term monitoring includes annual orthopedic examinations and radiographs to assess for osteoarthritis progression.

Pearls: 1) Early diagnosis and intervention are key to successful surgical outcomes. 2) The Ortolani sign is a reliable indicator of joint laxity. 3) PennHIP distraction index is a better predictor of osteoarthritis than OFA grading. 4) In TPO, the acetabular rotation should be 20-30 degrees to achieve optimal coverage. 5) In THR, proper implant sizing and positioning are critical to prevent luxation. Pitfalls: 1) Performing TPO in dogs with significant osteoarthritis leads to poor outcomes. 2) In FHNE, inadequate excision of the femoral neck can cause bone-on-bone contact and pain. 3) In THR, failure to secure the acetabular component can lead to loosening. 4) Overlooking concurrent orthopedic conditions, such as cruciate ligament rupture, can compromise surgical results.

Perioperative antimicrobial prophylaxis: cefazolin 22 mg/kg IV at induction and every 90 minutes during surgery. Postoperative analgesia: opioids such as hydromorphone 0.05-0.1 mg/kg IV q4-6h or fentanyl CRI 2-5 µg/kg/h for 24 hours. NSAIDs: carprofen 2.2 mg/kg PO q12h for 7-14 days, or meloxicam 0.1 mg/kg PO q24h. Local anesthesia: epidural morphine 0.1 mg/kg and bupivacaine 1 mg/kg for intraoperative and postoperative pain. Muscle relaxants: methocarbamol 15-20 mg/kg PO q8h as needed. Chondroprotectants: polysulfated glycosaminoglycan 5 mg/kg IM q7d for 8 weeks. For chronic pain, gabapentin 10 mg/kg PO q8h may be added.

Landmark studies include the PennHIP study by Smith et al. (1990) which established the distraction index as a predictor of osteoarthritis. The TPO procedure was popularized by Slocum and Slocum (1986), with studies showing 80-90% success rates in selected patients. THR has been extensively studied, with a meta-analysis by Skurla et al. (2000) reporting a 95% success rate. JPS was introduced by Dueland et al. (2001), showing that early intervention can improve hip conformation. Consensus guidelines from the ACVS and ECVS recommend early screening and surgical intervention for optimal outcomes. Recent studies focus on genetic markers and minimally invasive techniques.