FRACTURES OF THE TIBIA AND FIBULA
DAVID M. NUNAMAKER
Complete Fractures of the Tibia
Fractures of the tibia are common in the dog and may occur at any level. Fibular fractures usually occur with tibial fractures but on occasion can occur by themselves. Young dogs, because of their open physes, are more likely to have physeal fractures. Following physeal closure most fractures are then associated with the diaphysis of the bone.
BONY ANATOMY OF THE MATURE DOG AND CAT
The tibia is formed proximally by two flat condyles that make up the tibial plateau, which in turn articulates with the femoral condyles. The plateau is a triangular surface that is flat caudally and has a lateral edge and medial edge that converge cranially at the tibial tuberosity. Although the tibial plateau is quite broad, the actual articular surface is relegated to the posterior half of the proximal tibia. The cranial portion of the tibial plateau occupies the area associated with the fat pad. The tibial tuberosity, which serves as an insertion for the quadriceps muscle, is located just distally to the cranial border of the tibial plateau. The tuberosity continues distally as the tibial crest before tapering back to the diaphysis. The proximal tibial metaphysis is relatively flat medially but quite concave both caudally and laterally. All these surfaces blend into the distal tibial diaphysis, which is uniform in diameter but slightly S-shaped. It curves from medial to lateral in the proximal half and then back from lateral to medial in the distal half. When the diaphysis is viewed laterally an S-shape is also apparent, in which the concavity is proximal and caudal and the convexity is distal and caudal. The distal tibia is flared slightly and forms the distal articular surface. The distal epiphysis also forms the medial malleolus, which is the proximal attachment of the medial collateral ligament of the talocrural joint.
The fibula is a very thin bone. Its proximal end is enlarged to form the fibular head, which articulates with the lateral tibial condyle. It also serves as the distal attachment of the lateral collateral ligament of the stifle. The fibular diaphysis is very thin and possesses only a small medullary cavity, if any at all. Distally the diaphysis enlarges to the distal epiphysis, which forms the lateral malleolus, which articulates with the tibial tarsal bone. The malleolus also serves as the proximal attachment of the lateral collateral ligament of the hock(3) (Figs. 31-1 and 31-2). The normal bony anatomy of the tibia and fibula in the cat mimics that of the dog. The marrow cavity in the cat appears to be more uniform in diameter than it does in the dog, but no other important difference exists (Fig. 31-1C).
BONY ANATOMY OF THE IMMATURE DOG AND CAT
The tibia possesses three separate epiphyses of significance: the proximal epiphysis, the tibial tuberosity epiphysis, and the distal epiphysis. The proximal tibial epiphysis is relatively flat and forms the actual articular surface of the tibia. The epiphysis sits on the metaphysis, and the interface between the epiphysis and metaphysis contains two small concavities that help interdigitate the epiphysis to the metaphysis. The associated growth plate is responsible for approximately 40% of the length. Premature closure of the growth plate may result in tibial shortening or bowing if asymmetric closure occurs. At some point in development the proximal epiphysis is attached to the tibial tuberosity epiphysis. The tibial tuberosity epiphysis forms the prominence onto which the patellar ligament inserts. It has a flat interface with the metaphysis. Premature closure of the associated growth plate may cause extreme tibial deformity; that is, growing with a caudal convexity and cranial concavity. This growth plate normally closes relatively late in the development of the dog (Fig. 31-3). The distal tibial epiphysis interfaces the metaphysis via two triangular convexities on the metaphysis and corresponding concavities in the epiphysis. The growth plate is responsible for approximately 60% of the growth, and premature closure may cause tibial shortening. The fibula possesses an epiphysis both proximally and distally. Neither is of significance except for diaphyseal length.
FIG. 31-1 Line drawings representing two views of a mature normal tibia and fibula in a German shepherd (A), a dachshund (B), and a domestic short- haired cat (C). Figures illustrate comparative anatomy and are not drawn to the same scale.
FIG. 31-2 Cranial-caudal (A) and mediallateral (B) radiographic views of a normal tibia and fibula from a Great Dane.
BLOOD SUPPLY TO THE TIBIAL AND FIBULAR DIAPHYSIS In the mature dog and cat the major diaphyseal artery enters the tibia through the nutrient foramen in the posterior lateral edge of the proximal one-third diaphysis. The nutrient artery is a branch of the caudal tibial artery. Similarly a branch of the caudal tibial artery enters the fibular diaphysis.(3)
FIG. 31-3 Lateral radiograph of the proximal tibia of an immature, 9-month-old German shepherd. Note the width of the growth plate distal to the tibial tuberosity This area is slow to be mineralized and is often mistaken for fracture.