MECHANICAL FACTORS INFLUENCING THE HOLDING POWER OF SCREWS IN COMPACT BONE

¹ Institut de Recherches en Orthopedic, Verdum; Quebec, Canada

1. Cell differentiation around screws manufactured by two American and two Swiss companies and inserted into seventy femora in forty-one adult mongrel dogs has been observed over periods varying between two weeks and nine months.

2. This study reveals that, despite their excellent holding power, such screws are not everywhere in firm contact with the surrounding bone at the time of insertion. Indeed, only part of the thread surface facing the head of the screw touches the compact bone, all other surfaces being separated by a space up to 150 µ in thickness.

3. These spaces result both from the surgical technique employed and from the inaccurate measurements of drills, screws and taps.

4. Migrating cells invade these spaces during the first two weeks. In the absence of movement, these cells differentiate into osteogenic cells; movement leads to differentiation into fibroblasts, chondroblasts and osteoclasts, and failure of fixation ensues. In contrast, callus formation by osteogenic cells firmly anchors screws in four to five weeks, well before callus uniting the bone fragments has been established.

5. Extremities should be protected from undue stresses during those first few weeks after osteosynthesis, whatever the technique.

6. This study clearly demonstrates the importance oftesting screws in living bone to ascertain their holding power at all stages of fracture healing.