Bone in immature animals is biomechanically, anatomically, and physiologically distinct from mature bone. Failure to recognize the unique features of immature bone when treating fractures increases the risk of complications that may cause years of morbidity. Mature bone's inorganic mineral content accounts for 65 to 70% of its dry weight and gives bone its solid consistency and rigidity. The organic extracellular matrix, composed primarily of collagen, gives bone its flexibility and resiliency. The mineral content of bone rapidly increases during skeletal growth such that its stiffness increases up to 20 fold in the first 6 months. Compared to mature bone, immature bone is more ductile, absorbs more energy, and tolerates more strain and elastic deformation prior to fracture. Accordingly, incomplete "greenstick" fractures and bent (plastically deformed) bones are almost exclusively seen in growing dogs. The brittle nature of adult bone causes it to fracture with little plastic deformation such that anatomic reconstruction of bony segments is feasible when indicated. The more ductile nature of immature bone, however, can plastically deform quite significantly prior to fracture. Additionally, the soft nature of immature bone makes implants more prone to premature loosening.

Fractures in the growing dog often occur in the region of the physis. Unfortunately, rather than occurring in the hypertrophic zone as is typical in humans, naturally occurring physeal fractures in the canine often occur in the proliferative zone.¹ This may account for the relatively high risk of physeal dysfunction following injury in dogs. The effect of gonadectomy on physeal function should also be considered. Gonadectomy delays normal physeal closure and the earlier gonadectomy is performed, the more prolonged is the delay.²

The periosteum of growing dogs and cats is relatively thick and vascular and contributes dramatically to appositional bone growth and the rapid development of callus fracture healing. However, excessive emphasis on the fracture healing potential of growing dogs often distracts veterinary attention from the goal of rapid restoration of normal limb function.

Several general treatment strategies are applicable to growing dogs and cats:

 Focus upon rapid, full restoration of limb function in treatment selection rather than on fracture healing.

 Frequent convalescent recheck examinations with attentive observation of limb use and joint mobility and function.

 Do not span physes with implants that prevent longitudinal bone growth.

 Pins spanning a physis should be of as small a diameter as possible to achieve proper stability and should be positioned such that they can be removed when fracture union is achieved.

Pelvic fractures in puppies have an excellent prognosis for healing with most any treatment. However, severe mechanical constipation and secondary colo-rectal dysfunction may result if malunion causes excessive pelvic canal narrowing. Internal plate fixation of ilial fractures is performed when there is risk of pelvic collapse and the plate is contoured such that pelvic canal is opened to its normal dimension. When anatomic reconstruction of longitudinal ilial fractures is feasible, lag screws placed from ventral to dorsal alone or through a second bone plate reduces the risk of screw loosening by increasing the implant-bone interface and creating a tension band effect on the ventro-lateral tension band surface.^3-5

Femoral fractures in growing dogs and cats often occur at the physes, but also occur in the diaphysis. Slipped capital femoral epiphysis (SCFE) occurs in both dogs and cats. In cats, this condition often develops in overweight, neutered males between 1.5 and 2.5 years of age despite the lack of a traumatic incident and is theorized to be the result of chronic mechanical overload of the physis that is delayed on closure because of early gonadectomy.⁶ This condition may involve one or both hips. If only one hip is involved, the contralateral hip should be closely evaluated on radiographs and the pet owner informed that delayed development of the condition in the contralateral hip is not uncommon. In cats, SCFE can be effectively treated with internal fixation or femoral head/neck excision. In dogs, SCFE is most commonly the result of trauma, but nontraumatic cases have been identified.⁷ The risk of coxofemoral osteoarthritis is increased when SCFE develops in dogs < 4 months of age because physeal closure results in a shortened femoral neck. Normal femoral neck length and limb use are important in the normal development of the coxofemoral joint. Fixation of SCFE with multiple Kirschner wires is more stable than a single wire.⁸ Fixation with a lag screw is even more stable, but should be avoided if preservation of physeal growth is desired.⁹ Distal femoral physeal fractures are common in dogs and cats. Cats often develop Salter-Harris I fractures and dogs most commonly have Salter-Harris II fractures. Internal fixation of these fractures is easily performed with cross-pinning or dynamic pinning techniques. Cross-pinning provides superior to resistance to rotational forces, but either fixation provides adequate stability.¹⁰ A single intramedullary pin can be used if the interdigitation of the unique "four pegs in four cups" contour of the distal femoral physis provides adequate rotational stability. Femoral diaphyseal fractures often involve the distal half of the bone. While the prognosis for fracture union is excellent in properly treated fractures, the risk of quadriceps contracture should be assessed. Risk factors for quadriceps contracture include distal femoral fracture, extensive comminution or soft tissue injury, unstable fracture fixation, reduced stifle flexion upon fracture reduction/alignment, surgical stabilization combined with external coaptation. When there is increased risk of quadriceps contracture, a 90°/90° flexion sling should be used during the first 48-72 hours after surgery followed by passive/active physical therapy each day for the first 3-4 weeks after surgery. Attentive convalescent care should include recheck examinations every 2-3 days during the first two weeks following surgery.

Tibial fractures are relatively common in growing dogs and may occur at the physes or within the diaphysis. The tibial tubercle develops from a separate ossification center from the proximal tibial epiphysis. Avulsion fracture of the tibial tubercle may occur as an isolated injury or in combination with Salter-Harris I or II fractures of the proximal tibial physis. Tibial tubercle fractures may be treated with Kirschner wires or tension band fixation, though the latter is more likely to permanently close the physis. Salter-Harris fractures of the proximal tibial physis are often treated with multiple Kirschner wires. Radiographs are often made in 2 week intervals and implants are removed, if feasible, at the earliest sign of fracture union. Greenstick (incomplete) and minimally displaced fractures of the tibial diaphysis are relatively common in growing dogs. While coaptation is frequently effective in achieving bony union of such fractures, maintaining the stifle in some flexion, encouraging slow, controlled limb use and keeping the duration of coaptation to a minimum helps maintain retropatellar pressure and avoid the complication of patellar luxation.

References

1.  Johnson JM, Johnson AL, Eurell JA. Histological appearance of naturally occurring canine physeal fractures. Vet Surg 1994;23:81-86.

2.  Salmeri KR, Bloomberg MS, Scruggs SL, et al. Gonadectomy in immature dogs: effects on skeletal, physical, and behavioral development. J Am Vet Med Assoc 1991;198:1193-1203.

3.  Fitch R, Kerwin S, Hosgood G, Rooney M, et al. Radiographic evaluation and comparison of triple pelvic osteotomy with and without additional ventral plate stabilization in forty dogs--part 1. Vet Compar Orthop Traumatol 2002;15:164-171.

4.  VanGundy TE, Hulse DA, Nelson JK. Mechanical analysis of pelvic fractures. Vet Orthop Soc 1988;40.

5.  Vangundy TE, Hulse DA, Nelson JK, et al. Mechanical evaluation of two canine iliac fracture fixation systems. Vet Surg 1988;17:321-327.

6.  McNicholas WT Jr., Wilkens BE, Blevins WE, et al. Spontaneous femoral capital physeal fractures in adult cats: 26 cases (1996-2001). J Am Vet Med Assoc 2002;221:1731-1736.

7.  Moores AP, Owen MR, Fews D, et al. Slipped capital femoral epiphysis in dogs. J Small Anim Pract 2004;45:602-608.

8.  Belkoff SM, Millis DL, Probst CW. Biomechanical comparison of three internal fixations for treatment of slipped capital femoral epiphysis in immature dogs. Am J Vet Res 1992;53:2136-2140.

9.  Belkoff SM, Millis DL, Probst CW. Biomechanical comparison of 1-screw and 2-divergent pin internal fixations for treatment of slipped capital femoral epiphysis, using specimens obtained from immature dogs. Am J Vet Res 1993;54:1770-1773.

10. Sukhiani HR, Holmberg DL. Ex vivo biomechanical comparison of pin fixation techniques for canine distal femoral physeal fractures. Vet Surg 1997;26:398-407.