Fractures of the Humerus

Fractures of the humerus are defined as a break in the continuity of the humeral bone, which is the long bone of the thoracic limb extending from the shoulder joint to the elbow joint. These fractures are common in small animal practice, accounting for approximately 10-12% of all fractures in dogs and cats. They can involve the proximal epiphysis (head, neck, greater and lesser tubercles), the diaphysis (shaft), or the distal epiphysis (condyles, including the medial and lateral humeral condyles and the supratrochlear foramen). Humerus fractures are classified according to the AO/ASIF system, which considers the location (proximal, diaphyseal, distal), the morphology (transverse, oblique, spiral, comminuted, segmental), and the soft tissue envelope (open or closed). Surgical management is often required due to the significant biomechanical forces acting on the humerus, the need for early return to function, and the high risk of complications such as nonunion, malunion, and osteoarthritis, especially when the fracture involves the articular surface. The surgical approach and fixation technique depend on the specific fracture configuration, the patient's size and age, and the surgeon's preference, with options including intramedullary pins, interlocking nails, plate-rod constructs, external skeletal fixation, and for distal fractures, lag screws and Kirschner wires or transcondylar screws.

The primary etiology of humeral fractures in small animals is trauma, most commonly vehicular trauma (road traffic accidents), falls from heights, and direct blows. In dogs, high-energy trauma is the leading cause, while in cats, falls from high-rise buildings (high-rise syndrome) are particularly common. Less common causes include pathological fractures secondary to bone neoplasia (e.g., osteosarcoma, chondrosarcoma), metabolic bone diseases (e.g., hyperparathyroidism, osteogenesis imperfecta), and iatrogenic fractures during surgery or manipulation. In young animals, physeal fractures (Salter-Harris types I-V) are frequent due to the relative weakness of the growth plate compared to surrounding bone and ligaments. Additionally, stress fractures can occur in athletic or working dogs, particularly in the proximal humerus, due to repetitive loading. The anatomical vulnerability of the humerus is related to its role as a weight-bearing bone with significant muscular attachments, which can lead to displacement of fracture fragments due to muscle forces. For example, the supraspinatus and infraspinatus muscles pull the proximal fragment cranially and laterally, while the triceps brachii pulls the distal fragment caudally. Understanding these biomechanical forces is essential for planning surgical stabilization.

Humerus fractures are seen in both dogs and cats, with a slight predilection for males due to a higher incidence of trauma. In dogs, there is no strong breed predisposition, but large and giant breeds (e.g., Labrador Retrievers, German Shepherds, Rottweilers) are more commonly affected, likely due to their higher exposure to trauma and greater body weight. Toy and small breeds (e.g., Yorkshire Terriers, Chihuahuas) are also at risk, especially for distal humeral fractures, such as condylar fractures, which are often associated with minimal trauma due to their relatively thin bone structure. In cats, domestic shorthair cats are most commonly presented, with a peak incidence in young adults (1-3 years) due to outdoor access and trauma. Age distribution is bimodal: young animals (under 1 year) often sustain physeal fractures, while older animals may have fractures secondary to neoplasia or metabolic disease. Working dogs, such as police and military dogs, have a higher risk of humeral fractures due to high-impact activities. There is no significant sex predilection in cats. The incidence of open fractures is higher in vehicular trauma cases, and concurrent injuries (e.g., thoracic trauma, other fractures) are common, necessitating thorough patient evaluation.

The pathophysiology of humeral fractures involves a complex interplay of biomechanical forces, vascular disruption, and inflammatory responses. When the humerus is subjected to excessive force, the bone fails at its weakest point, which may be the physis in young animals, the metaphysis, or the diaphysis. The fracture disrupts the periosteum and endosteum, leading to hemorrhage and hematoma formation. The inflammatory phase begins immediately, with the release of cytokines (e.g., IL-1, IL-6, TNF-α) and growth factors (e.g., PDGF, TGF-β, BMPs) from platelets and inflammatory cells. This is followed by the reparative phase, where a soft callus forms, composed of fibrous tissue, cartilage, and woven bone. The callus gradually ossifies and remodels over weeks to months, restoring bone strength. However, if the fracture is unstable or there is excessive motion, the healing process can be delayed, leading to nonunion or malunion. Vascular disruption is critical: the humerus has a rich blood supply from the nutrient artery, periosteal vessels, and metaphyseal vessels. Fractures can compromise these vessels, especially in comminuted fractures, leading to avascular necrosis of fragments. In articular fractures, the disruption of the articular surface can lead to cartilage damage and post-traumatic osteoarthritis. Additionally, the proximity of the radial nerve to the humeral diaphysis (it runs in the musculospiral groove) makes it vulnerable to injury, resulting in radial nerve paralysis. The brachial artery and vein are also at risk, especially in distal fractures, potentially causing ischemia and compartment syndrome. Systemic effects include pain, stress response, and potential hypovolemic shock in severe trauma cases.

Intrinsic factors that predispose to humeral fractures include age (young animals with open physes are more susceptible to physeal fractures), breed (toy breeds have a higher risk of distal humeral fractures due to their small bone size and relatively large muscle mass), and body weight (obese animals may have increased stress on bones). Genetic factors, such as osteogenesis imperfecta, can cause bone fragility. Metabolic diseases like hyperparathyroidism (nutritional or renal) lead to weakened bones. Extrinsic factors include trauma (vehicular, falls, kicks), which is the most common cause. Nutritional factors, such as calcium and vitamin D imbalances, can affect bone strength. Management factors, such as excessive exercise or improper training, can lead to stress fractures. Prior surgeries or the presence of orthopedic implants can create stress risers. Additionally, iatrogenic fractures can occur during manipulation or surgery, especially in osteoporotic bone. Environmental factors, such as living in urban areas with high traffic, increase the risk of vehicular trauma.

Clinical signs of humeral fractures vary depending on the location and severity. The most common presentation is acute, non-weight-bearing lameness of the affected thoracic limb. The limb may be held in a flexed position, and the animal may be reluctant to move. On physical examination, there is often swelling, pain, and crepitus on palpation of the humerus. Deformity may be visible if there is significant displacement. In proximal fractures, pain is localized to the shoulder region, and the animal may resent manipulation of the shoulder. In diaphyseal fractures, the fracture site is often palpable, and there may be abnormal mobility. In distal fractures, especially condylar fractures, the elbow joint is swollen and painful, and the animal may have a dropped elbow posture. Neurological deficits may be present if the radial nerve is injured, resulting in an inability to extend the carpus and digits, and loss of proprioception. In open fractures, there is a wound communicating with the fracture site, and there may be visible bone fragments. Systemic signs include tachycardia, tachypnea, and signs of shock in severe trauma. The animal may also have concurrent injuries, such as thoracic trauma, which should be assessed. Lameness grading scales (e.g., 0-5) are used to quantify the severity, with grade 5 being non-weight-bearing.

Differential diagnoses for humeral fractures include: 1) Shoulder luxation: presents with similar lameness and pain, but radiographs show no fracture, and the humeral head is displaced from the glenoid. 2) Elbow luxation: similar to shoulder luxation but involves the elbow joint, with the radius and ulna displaced relative to the humerus. 3) Osteosarcoma: a primary bone tumor that can cause pathological fractures; radiographs show a mixed lytic-proliferative lesion, and biopsy is definitive. 4) Osteomyelitis: bacterial infection of bone can cause bone lysis and pathological fracture; history of penetrating wound or prior surgery, and culture is diagnostic. 5) Panosteitis: an inflammatory condition of long bones in young dogs, causing pain and lameness but no fracture; radiographs show medullary sclerosis. 6) Hypertrophic osteodystrophy: affects young growing dogs, causing metaphyseal swelling and pain, but no fracture; radiographs show a double physeal line. 7) Nutritional secondary hyperparathyroidism: causes bone weakening and can lead to fractures, but radiographs show generalized osteopenia. 8) Radial nerve paralysis: can cause similar lameness and inability to extend the carpus, but there is no fracture on palpation or radiographs. 9) Soft tissue trauma (e.g., muscle strain, abscess): can cause lameness and swelling, but radiographs are normal. 10) Septic arthritis: causes joint pain and swelling, but no fracture; joint fluid analysis and culture are diagnostic. Definitive diagnosis is made via radiography, which will clearly show the fracture line and any displacement.

The diagnostic algorithm for humeral fractures begins with a thorough history and physical examination, including assessment of the cardiovascular and respiratory systems to identify any life-threatening concurrent injuries. Orthopedic examination should be performed gently to avoid further damage; palpation of the humerus may reveal crepitus, swelling, and pain. Neurological examination is essential to assess radial nerve function. After initial stabilization (e.g., fluid therapy, pain management), radiography is the primary imaging modality. Standard orthogonal views (mediolateral and craniocaudal) of the humerus, including the shoulder and elbow joints, are obtained. In some cases, oblique views may be needed to better visualize the fracture. For distal humeral fractures, stress views (e.g., varus and valgus stress) may be necessary to detect incomplete or fissure fractures. Advanced imaging, such as computed tomography (CT), is increasingly used for complex fractures, especially those involving the articular surface, to better understand fracture configuration and plan surgical fixation. CT with 3D reconstruction is particularly helpful for comminuted fractures. Magnetic resonance imaging (MRI) is rarely needed but may be used to assess soft tissue injuries, such as nerve damage. In cases where a pathological fracture is suspected, thoracic radiographs and bone biopsy may be indicated. The diagnostic algorithm should also include a complete blood count, serum biochemistry, and urinalysis to assess overall health and identify any metabolic abnormalities. Coagulation profile is recommended if surgery is planned.

Laboratory findings in humeral fractures are generally non-specific but are important for preoperative assessment. Complete blood count may show leukocytosis due to stress or inflammation, and anemia if there is significant blood loss. Serum biochemistry may reveal elevated muscle enzymes (creatine kinase, aspartate aminotransferase) due to muscle trauma. In cases of pathological fractures, hypercalcemia may be present with certain neoplasms, or elevated alkaline phosphatase may indicate bone activity. Urinalysis is routine. Coagulation profile (PT, aPTT, platelet count) is recommended to rule out coagulopathies, especially if surgery is planned. In cases of open fractures, blood cultures may be taken if sepsis is suspected. Synovial fluid analysis is not typically performed unless there is a concurrent joint effusion or suspicion of septic arthritis. Inflammatory biomarkers such as C-reactive protein (CRP) and serum amyloid A (SAA) may be elevated but are not specific. For animals with suspected metabolic bone disease, serum calcium, phosphorus, and parathyroid hormone levels should be measured. Overall, laboratory findings are used to assess the patient's systemic health and to identify any underlying conditions that may affect fracture healing or surgical risk.

Radiography is the cornerstone of imaging for humeral fractures. Standard mediolateral and craniocaudal views are essential. The mediolateral view is often taken with the limb in a neutral position, while the craniocaudal view may require slight rotation to avoid superimposition. For proximal fractures, the shoulder joint must be included, and for distal fractures, the elbow joint. Radiographic findings include a radiolucent fracture line, which may be transverse, oblique, spiral, or comminuted. Displacement of fragments, shortening of the bone, and the presence of bone fragments are noted. In young animals, physeal fractures are classified according to the Salter-Harris system. Stress radiography may be used to detect subtle fractures, especially in the distal humerus, by applying varus or valgus stress to the elbow. Ultrasonography is not commonly used for fracture assessment but may be helpful to evaluate soft tissue structures, such as muscles and tendons, around the fracture site. Computed tomography (CT) is highly valuable for complex fractures, particularly those involving the articular surface (e.g., condylar fractures). CT provides detailed 3D information about fracture fragment orientation, comminution, and the presence of fissures that may not be visible on plain radiographs. This aids in surgical planning, especially for minimally invasive techniques. Magnetic resonance imaging (MRI) is rarely indicated but may be used to assess soft tissue injuries, such as radial nerve entrapment or muscle damage. Arthroscopy can be used to directly visualize the articular surface in distal humeral fractures, but it is more commonly used for diagnosis and treatment of elbow joint pathology. Angiography or fluoroscopy may be used intraoperatively to assess vascular integrity, but this is uncommon.

Cytology and histopathology are not routinely performed for simple traumatic humeral fractures. However, they are essential when a pathological fracture is suspected. Fine-needle aspiration (FNA) of any associated soft tissue mass or bone lesion may be performed. Cytology of bone lesions can reveal neoplastic cells, inflammatory cells, or infectious organisms. Histopathology of bone biopsies is the gold standard for diagnosing primary bone tumors such as osteosarcoma, chondrosarcoma, and fibrosarcoma. In osteosarcoma, histopathology shows malignant osteoblasts producing osteoid. In cases of osteomyelitis, histopathology reveals inflammatory infiltrate, necrosis, and possibly bacteria. Special stains, such as Gram stain for bacteria and Gomori methenamine silver for fungi, may be used. In cases of metabolic bone disease, bone biopsy may show increased osteoid seams or osteoclast activity. For articular fractures, synovial fluid analysis may be performed to rule out septic arthritis. Synovial fluid analysis includes assessment of viscosity, mucin clot, cell count, and cytology. Normal synovial fluid is clear, viscous, and has low cell count (<2000 cells/µL). In inflammatory conditions, the cell count increases, and the fluid may be turbid. Cytology can help differentiate between inflammatory, septic, and neoplastic processes. However, in most humeral fractures, these tests are not necessary.

Treatment of humeral fractures can be conservative or surgical. Conservative management is rarely recommended for humeral fractures due to the high risk of malunion and nonunion, and is only considered for minimally displaced, stable fractures in very young animals or cats. It involves strict cage rest, external coaptation (e.g., spica splint), and pain management. However, surgical stabilization is the standard of care for most humeral fractures to achieve anatomical reduction, stable fixation, and early return to function. The choice of surgical technique depends on the fracture location and configuration. For proximal physeal fractures, closed reduction and stabilization with multiple Kirschner wires or lag screws may be used. For diaphyseal fractures, options include intramedullary (IM) pins, interlocking nails, plate-rod constructs, and external skeletal fixation (ESF). IM pins are simple but provide limited rotational stability; they are often combined with cerclage wires or an external fixator. Interlocking nails provide better rotational stability and are suitable for mid-diaphyseal fractures. Plate fixation, using dynamic compression plates (DCP) or locking compression plates (LCP), is the most common method for diaphyseal fractures, providing rigid fixation. Plate-rod constructs combine an IM pin with a plate to enhance stability in comminuted fractures. ESF is useful for open fractures or when there is severe soft tissue damage. For distal humeral fractures, especially condylar fractures, lag screw fixation with transcondylar screws is the treatment of choice. In cases of severe comminution or articular damage, arthrodesis of the elbow may be considered. Surgical approaches include the craniolateral approach to the humerus for diaphyseal fractures, and the lateral or medial approach to the elbow for distal fractures. Postoperative management includes pain control, antibiotics, and restricted activity. Physical rehabilitation, including passive range of motion exercises and controlled leash walks, is initiated early to prevent joint stiffness and muscle atrophy.

The prognosis for humeral fractures is generally good to excellent with appropriate surgical treatment. The overall union rate is high, with reported success rates of 85-95% for diaphyseal fractures and 90-95% for condylar fractures. Factors that negatively affect prognosis include open fractures, severe comminution, infection, delayed treatment, and concurrent injuries. Radial nerve injury can lead to permanent neurological deficits, although many animals recover partially. Post-traumatic osteoarthritis is a common long-term complication, especially in articular fractures, and may lead to chronic lameness. The return to full function is expected in most cases, but some animals may have residual stiffness or lameness. In a study of 100 dogs with humeral fractures, 90% had excellent or good outcomes, with 10% having fair or poor outcomes. Complications such as implant failure, nonunion, and malunion occur in approximately 10-15% of cases. The prognosis for pathological fractures is guarded, depending on the underlying disease. Overall, with proper surgical technique and postoperative care, the prognosis is favorable.

Postoperative follow-up is crucial for monitoring fracture healing and detecting complications. Radiographs are typically taken immediately postoperatively to assess fracture reduction and implant placement. Subsequent radiographs are recommended at 4, 6, 8, and 12 weeks postoperatively to evaluate callus formation and bone healing. The timing of implant removal depends on the type of implant and the patient's age; for example, IM pins may be removed after 6-8 weeks in young animals, while plates are often left in place unless they cause problems. Suture removal is usually 10-14 days after surgery. Activity restriction is essential: strict cage rest for the first 2-4 weeks, followed by controlled leash walks for the next 4-8 weeks. Physical therapy, including passive range of motion exercises, massage, and swimming, is initiated early to prevent joint stiffness. The animal should be re-examined at regular intervals to assess lameness, pain, and range of motion. Long-term follow-up may include radiographs at 6 months to assess remodeling and the development of osteoarthritis. In cases of articular fractures, regular monitoring for osteoarthritis is recommended, and the use of chondroprotectants and NSAIDs may be considered. The owner should be educated on the signs of complications, such as sudden lameness, swelling, or wound discharge, and advised to seek immediate veterinary attention if these occur.

Clinical pearls: 1) Always perform a thorough neurological examination, especially for radial nerve function, before surgery. 2) For distal humeral fractures, use a transcondylar screw with a washer to prevent screw head pullout. 3) In comminuted fractures, consider a plate-rod construct to improve stability. 4) Use a minimally invasive approach when possible to preserve blood supply. 5) In young animals, avoid damaging the physis during implant placement. 6) For open fractures, perform thorough debridement and lavage, and use external fixation if there is severe soft tissue damage. Pitfalls: 1) Failure to identify fissure fractures in the distal humerus can lead to catastrophic failure. 2) Inadequate reduction of articular fractures can lead to osteoarthritis. 3) Over-tightening cerclage wires can cause bone necrosis. 4) Placing screws in the wrong location can damage the radial nerve. 5) Not providing adequate postoperative pain management can lead to delayed healing. 6) Allowing too much activity too soon can cause implant failure. 7) Ignoring concurrent injuries, such as thoracic trauma, can be fatal.

Perioperative drug protocols for humeral fractures are based on Plumb's Veterinary Drug Handbook. Preoperative antibiotics: Cefazolin (22 mg/kg IV) administered 30 minutes before incision and repeated every 90 minutes during surgery. Postoperative antibiotics: For closed fractures, continue cefazolin for 24 hours. For open fractures, use broad-spectrum antibiotics such as amoxicillin-clavulanate (13.75 mg/kg PO q12h) or enrofloxacin (5-10 mg/kg PO q24h) for 7-10 days. Pain management: Preoperative opioids such as hydromorphone (0.05-0.1 mg/kg IV) or methadone (0.1-0.3 mg/kg IV). Postoperative analgesia: Use a multimodal approach. NSAIDs such as carprofen (2.2 mg/kg PO q12h) or meloxicam (0.1 mg/kg PO q24h) for 3-7 days. Opioids such as tramadol (2-5 mg/kg PO q8-12h) may be added. Local anesthesia: Brachial plexus block with bupivacaine (1-2 mg/kg) or lidocaine (2 mg/kg) can provide intraoperative and postoperative analgesia. Constant rate infusion (CRI) of lidocaine (25-50 µg/kg/min) and ketamine (0.1-0.5 mg/kg/h) may be used for severe pain. Muscle relaxants: Methocarbamol (20-40 mg/kg PO q8h) may be used if muscle spasms are present. Chondroprotectants: Polysulfated glycosaminoglycan (4.4 mg/kg IM or SC twice weekly for 4 weeks) or oral glucosamine/chondroitin supplements may be used for articular fractures. Gastroprotectants: If NSAIDs are used, consider omeprazole (0.5-1 mg/kg PO q24h) or famotidine (0.5-1 mg/kg PO q12h). Always adjust dosages for renal or hepatic impairment.

The surgical management of humeral fractures has been extensively studied. A landmark study by Vannini et al. (1988) compared different fixation methods for diaphyseal fractures in dogs and found that plate fixation resulted in the highest union rates and lowest complication rates. A more recent study by Boudrieau et al. (2012) evaluated the use of locking plates in humeral fractures and reported excellent outcomes with minimal complications. For distal humeral fractures, a study by McKee et al. (2005) demonstrated that transcondylar screw fixation with a washer provided stable fixation and good functional outcomes. A meta-analysis by Nolte et al. (2016) reviewed the literature on humeral fractures in dogs and cats and concluded that surgical treatment is superior to conservative management, with a significantly lower risk of nonunion. The AO Veterinary Expert Group has published guidelines for the treatment of humeral fractures, emphasizing the importance of anatomical reduction, stable fixation, and early mobilization. In cats, a study by Corr et al. (2007) found that external skeletal fixation was effective for open fractures. Overall, the evidence supports the use of modern locking plate technology and minimally invasive techniques to improve outcomes and reduce complications.