This lecture will focus on the immature dog and fractures specific to them. Included will be common fractures and their surgical fixation, the evidence on healing times, and when and if to remove implants.

Physeal fractures occur in immature animals with open physes, usually secondary to trauma, though this may be minimal depending on the affected location. It is often presumed that regardless of the implant the physis may close as a result of the inciting or surgical trauma.

Fixation techniques for physeal fractures should minimally impact blood supply, result in accurate and stable reduction, be easily removable, and not significantly further damage the physis. Implants that prevent further physeal growth, such as bone plates, lag screws, and external skeletal fixators, should be avoided in the growing animal. In the rare case in which the fracture configuration requires bridging of the physis with of one of these implants, early removal as soon as two weeks after surgery should be considered depending on the degree of healing. They should not be in place longer than 4 weeks.

The most commonly used implants for physeal fractures are pins. Pins placed parallel to each other offer biological and mechanical advantages in comparison to divergent pins. Parallel pins allow for continued physeal growth, while divergent pins may create a locking effect on the physis, resulting in premature closure. Mechanically, the forces applied to parallel pins are distributed equally between the pins, while with divergent pins, uneven distribution of loads between the implants renders the technique weaker and predisposes the repair to failure. Parallel pins may be inappropriate in some situations as discussed below.

Fracture Locations

Proximal Humerus

Fractures of the humeral head are usually Salter-Harris type I and II. The humeral head or the greater tubercle can separate independently, but more commonly the two epiphyses can stay confluent.

The most useful approach for these fractures is a combined approach to the craniolateral region of the shoulder joint and the proximal humerus. Little additional dissection is usually required because these frequently separate during the initial injury. Reduction is aided by placing the shoulder in extension.

If the two fragments (the humeral head and the greater tubercle) are still together, adequate stabilization is achieved using two pins from the proximal aspect of the greater tubercle into the humeral neck or the proximal diaphysis. These are placed in parallel fashion if further growth is expected to minimize compression and allow for continued physeal growth. In animals at or near skeletal maturity, a wider variety of fixation devices, including lag screws and tension band wires, can be used to provide additional stability.

Salter-Harris type V or VI fractures have also been reported in the proximal humeral physis. These fractures may result in premature closure and shortening of the bone or bowing if closure is asymmetric.

With appropriate owners and post-operative restrictions, external coaptation is not required. Restrictions are usually in place for 4 weeks, but with uncomplicated healing, the function outcome of these fractures is usually very good.

Lateral/Bicondylar Humeral Condylar Fractures

The fracture lines in humeral condylar fractures always extend through the joint surface and then through one or both epicondyles or epicondylar crests, or even into the distal shaft.

Lateral condylar fractures occur more frequently than medial or bicondylar fractures due to anatomic and biomechanical differences. They are usually associated with low energy trauma such as jumping, and mainly affect patients younger than 1 year of age with a peak at 4 months of age. Typically classified as Salter-Harris type IV fractures, these fractures communicate with the articular surface and involve the distal humeral growth plate. Salter-Harris type III fractures may also occur.

Anatomic reduction and rigid internal fixation is indicated for any articular fracture. Failure to repair a lateral condylar fracture results in medial subluxation of the elbow and joint deformity due to mal- or nonunion. Repair of these fractures has been reported by both open and closed, fluoroscopically guided techniques. Standard fixation includes a transcondylar lag screw, with or without an antirotational pin or screw. The antirotational pin engages the epicondylar crest. The transcondylar screw should have at least one thread protruding from the medial cortex for optimal bone purchase. In immature dogs, a metal washer should be used to distribute the force more evenly on the thin shell of cortical bone. In addition, the use of Kirschner wires alone in young dogs weighing less than 4 kg and the use of self-compressing pins and a positional transcondylar screw have all been reported.

Studies have shown that there is no decrease in humeral length after lateral condylar fractures, even when the implants crossed the growth plate. Therefore, implant removal to allow for further growth may be unnecessary. The likely reason for this is the fact that the distal growth plate of the humerus is only responsible for 20% of the length of the humerus and closes at around 5 to 8 months of age. The proximal growth plate closes much later and is responsible for the remainder of growth.

It is interesting to note that the accuracy of fracture reduction apparently does not correlate with follow-up osteoarthritis score, peak vertical force, or vertical impulse from force platform analysis, nor does it correlate with range of elbow joint motion. Owners should be told that osteoarthritis is expected and will progress in all elbows.

If this fracture is seen in older dogs, particularly after minimal trauma, then incomplete ossification of the humeral condyle (IOHC) should be considered.

Femur

Capital Physeal

Capital physeal femoral fractures are typically classified as Salter-Harris type I. Concurrent separation of the trochanteric physis can be seen in 11% to 15% of cases. Surgical treatment is recommended as soon as possible, as any delay results in ongoing trauma to the local vasculature and physeal surfaces.

Open or closed approaches have been reported for the reduction and internal fixation of these fractures. For the open approach, surgeons usually chose a craniolateral, dorsal, or ventral approach to the hip joint. If a dorsal approach is chosen, the gluteal tenotomy should only be used in animals younger than 3 to 5 months of age; otherwise, osteotomy of the greater trochanter is appropriate. Closed reduction with intraoperative fluoroscopy may limit iatrogenic trauma. The L-shaped profile of the capital physis helps guide reduction and provides some intrinsic stability that counteracts shear forces after reduction.

Multiple Kirschner wires or small-diameter Steinmann pins are usually used to prevent compression and iatrogenic closure of the physis and can be placed normograde or retrograde. The pin diameter depends on the patient size; Kirschner wires (0.7 to 1.6 mm diameter) are generally used in cats and in many small- and medium-breed dogs. Small-diameter Steinmann pins may be indicated for large- or giant-breed dogs.

Distal-to-proximal normograde placement is the least invasive, but achieving appropriate accuracy can be technically challenging. Pin insertion begins on the lateral aspect of the femur, caudodistal to the greater trochanter at the level of the third trochanter. The pin is angled to exit near the center of the capital physis. Subsequent pins are placed in a similar fashion. After anatomic reduction is achieved, the pins are advanced and secured in the proximal segment. The limited bone stock proximal to the physis requires that pins be advanced as deeply as possible within the capital epiphysis. If the pins penetrate articular cartilage, severe chondrolysis and degenerative joint disease will develop. Retrograde pinning may provide more accurate pin placement, but this is offset by the extensive dissection, which can result in subsequent iatrogenic injury of the blood supply and physis.

The number of pins used does not seem to affect the development of femoral neck resorption or osteoarthritis. Regardless of the patient's size, a minimum of two pins is recommended to resist rotational and shear forces, but no more than three pins are considered necessary. Implant failure is not a common problem in these fractures and additional pins may only cause complications.

Resorption of the femoral neck, 3–6 weeks following open reduction and internal fixation, has been reported in up to 70% of cases. Damage to the vascular network supplying the femoral head and neck and overfixation may be responsible. The resultant narrowing and occasional shortening of the neck tend to be self-limiting, and subsequent collapse is rare.

Fractures of the Greater Trochanter

These avulsion fractures are frequently seen with concurrent fracture of the capital physis or femoral neck. If the displacement is minimal, strict cage rest for 3 to 4 weeks may be enough to permit acceptable healing without loss of function. If significant displacement is present, surgery is indicated to maintain normal function of the limb.

Primary repair involves open reduction via a craniolateral approach to the hip joint and stabilization using a pin and tension band system. The outcome is usually good. The pins are placed perpendicular to the physis and parallel to each other: this converts tensile forces acting on the greater trochanter into compressive forces along the fracture line. The compression achieved can prevent further growth from the trochanteric physis. This does generally not affect the length of the femur, but it may alter hip joint biomechanics and predispose to degenerative joint disease.

Distal Femoral

Fractures of the distal femoral physis are generally classified as a Salter-Harris type II fracture.

Again, reduction and stabilization are important to prevent further physeal trauma and preserve remaining growth potential. Both open and closed reduction can be performed, but closed reduction may be limited to acute fractures due to muscular contraction and periarticular fibrosis. Slight overreduction of the epiphysis with respect to the metaphysis allows for greater distal implant purchase. Overreduction is functionally acceptable and has not been associated with major complications. Underreduction and varus or valgus malalignment, on the other hand, are associated with greater risks of implant migration, implant failure, patellar impingement or luxation, and malunion.

The physeal interdigitations alone are not sufficient to counteract the forces acting on the physis. Surgical techniques usually include a combination of at least two Kirschner wires or small-diameter Steinmann pins. These are usually placed in distal to proximal normograde fashion. Distally, the pins begin on the nonarticular surfaces of the condyles, just caudal to the medial and lateral aspects of the femoral trochlea. The challenge with this technique is insertion of the pins at an appropriate angle so their cross-over point is above the fracture line. Alternatively, retrograde insertion proximal to distal within the distal fragment maximizes accurate pin placement, but requires a more invasive surgical approach. In addition, it is associated with increased risk of articular cartilage penetration.

With appropriate surgical fixation, the prognosis for normal limb function following a type I or II Salter-Harris fracture is good to excellent, but frequently results in premature physeal closure. This is likely related to the inciting trauma. The distal femoral physis accounts for 75% of femoral growth; therefore, premature closure can be detrimental in large breed dogs less than 6 months of age. The clinical importance of this may not be significant, as overall limb length is maintained through compensatory increase in joint angles. However, asymmetric closure secondary to unicondylar fracture may affect limb alignment in valgus or varus, and can result in subsequent osteoarthritis, as well as patellar luxation.