The primary displacement in rotation of the femoral head, the rotation around its axis, occurs in general when the interdigitation of the fracture surfaces fails to materialize (multifragmentary or comminuted fracture). It is difficult to diagnose as tilting in varus and antecurvature dominate the picture.
The secondary displacement in rotation happens when the stability of a neck fracture has been misinterpreted, when an insufficient number of implants has been used or when the implants were not strong enough to neutralize the deforming forces during walking. In such an instance radiographs show a change in position of the femoral head or a loss of reduction not only in varus and antecurvature but also a changed position of the screws in relation to each other. This loss of reduction appears to cause on a.-p. films a convergence of screws that were initially parallel and on lateral films a divergence. Even a crossing of screws may be present (see Figs. 94, 185 and 186).
The understanding of fracture healing has recently undergone a marked change. Methods of internal fixation that lead to the formation of a pe-riosteal callus induced by micromovements (such as intramedullary nailing) have proven to be biologically and biomechanically more favorable than rigid internal fixation (such as DC plates) that causes a consolidation by a primary angiogenic callus formation. The intracapsular neck, however, represents an exception as it is not covered by a periosteum. For this reason no periosteal callus can form after fracture except in vertical fracture where the fracture line reaches beyond the capsule. In the absence of periosteal callus already small rotational movements may have unfavorable consequences.
The diagnosis of a primary loss of reduction in rotation is difficult. Contact between fragments in multifragmentary fractures is usually not complete. Consequently, it is difficult on standard radiographs to determine up to which point the deviation between the main fragments from the oval neck shape is the consequence of secondary displace ment of small fragments or of a secondary displacement in rotation. During the assessment of lateral radiographs it is useful to look carefully at the usually less damaged anterior cortex. Kyle et al (1994) have shown that a primary displacement in rotation can be recognized by a deviation of the course of trabeculae. This requires a.-p. radiographs of excellent quality that are often difficult to obtain on account of senile osteoporosis (Fig. 157).
Principal compressive trabeculae iT
Fig. 157. Diagnosis of a rotational displacement of the femoral neck based on a changed trabecular orientation (Kyle et al, 1994)
The postoperative secondary loss of reduction in rotation can be prevented in the majority of cases with a standard double screw fixation complemented by a 2 mm two-hole plate attached to the caudal screw. In the presence of marked instability (severe osteoporosis, multifragmentary or comminuted fractures) the following modifications (possibly in combination) protect also against rotation when using the methods described earlier:
- Use of three screws,
- Attachment of both screws to one two-hole plate each;
- Fixation of both screws with a 2 mm three-hole plate anchoring both screws;
- Fixation of both screws with a 2 mm three-hole plate anchoring both screws and additional fixation of the caudal screw with a two-hole plate (see Fig. 165);
- Use of screws with a greater thread diameter (9.5 mm);
- DCD plates (particularly satellite plates and double angle-stable DCD plates);
- Use of flanged screws.
The true rotation of the femoral head should not be confused with the torsion of the head occurring during the reduction of a displaced neck fracture. The latter displacement is usually corrected by internal rotation of the lower limb around the longitudinal axis of the femur (the antecurvature seen in lateral radiographs). In this instance the caudal fragment is adapted to the cranial one, this does not produce a rotation around the neck axis: the neck stump does not turn but rather tilts medially and posteriorly. The insufficient internal rotation is recognized in the lateral film by a remaining antecurvature and a posterior translation. The caudal fragment, the stump of the neck, lies further anterior. The more frequently occurring increased internal rotation indicates a recurvatum and an anterior translation. The neck stump is seen posteriorly. Here we are dealing with gross errors of reduction. Due to the missing adaptation between the fragments a loss of reduction is imminent. The tension of the posterior capsular vessels seen in instances of recurvatum increases the risk of circulatory disturbances (see Sect. 7.2).
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