INJURIES OF THE PATELLA
Fracture of the patella is a comparatively common injury in adult males. Most frequently it is due to muscular action the patella being snapped across the lower end of the femur by a sudden and forcible contraction of the quadriceps extensor muscle while the limb is partly flexed—as, for example, in the attempt to avoid falling backward. The bone is then broken as one breaks a stick by bending it across the knee, and the line of fracture, which is transverse or slightly oblique, crosses the bone a little below its middle. Fractures produced in this way are almost never compound.
Fig. 87.—Radiogram of Fracture of Patella.
The degree of displacement of the fragments depends upon the extent to which the expansion of the quadriceps tendon is lacerated. As a rule, it is but slightly torn, so that the separation of the fragments does not exceed an inch. In other cases it is widely torn, and the contraction of the quadriceps muscle is then able to separate the fragments by three or four inches, and sometimes causes tilting of the upper fragment. The blood effused into the joint tends still further to increase the separation. As the periosteum is usually torn at a level lower than the fracture, its free margin hangs as a fringe from the proximal fragment, and by getting between the broken ends may form a barrier to osseous union (Macewen).
Fig. 88.—Fracture of Patella, showing wide separation of fragments, which are united by a fibrous band.
(Anatomical Museum of the University of Edinburgh.)
Clinical Features.—Immediately the bone breaks, the patient falls, and he is unable to rise again, as the limb is at once rendered useless, and in attempting to do so we have known him to fracture the patella of the other limb. The power of extending the limb is lost, and the patient is unable to lift his foot off the ground. The knee-joint is filled with blood and synovia, which usually extend into the bursa under the quadriceps. The two fragments can be detected, separated by an interval which admits of the finger being placed between them, and which is increased on flexing the knee. On relaxing the quadriceps, the fragments may be approximated more or less completely.