Background: Recent reports have implicated cubitus varus deformity
as a risk factor for the development of late posterolateral rotatory
instability of the elbow. The purpose of this study was to determine the
biomechanical relationship between cubitus varus and strain in the lateral
ulnar collateral ligament and subluxation of the ulnohumeral joint as it
relates to posterolateral rotatory instability.
Methods: Eleven fresh-frozen cadaveric elbow joints were mounted in
an apparatus that allowed active elbow motion. Supracondylar osteotomies were
performed to create cubitus varus deformities of 0° to 30° in 5°
increments. Each elbow was loaded with a supination moment of 1.2 Nm and a
resistive triceps extension force. Strain in the lateral ulnar collateral
ligament and widening of the ulnohumeral joint were measured at each position
of varus, and the resulting strain and joint-widening values were
averaged.
Results: Strain in the lateral ulnar collateral ligament was found
to increase as the cubitus varus deformity increased. Likewise, the
ulnohumeral joint space was seen to progressively widen with increasing
cubitus varus. Strain was significantly increased at 30° of varus
deformity (p = 0.03), and widening of the ulnohumeral joint space was
significantly increased at 25° of varus deformity (p = 0.004). When
differences in the size of the cadaveric specimens were accounted for in an
analysis of covariance, ligament strain was significantly increased at 25°
of varus (p = 0.005) and widening of the ulnohumeral joint space, at 20°
(p = 0.01).
Conclusions: Cubitus varus deformity of the elbow increases strain
in the lateral ulnar collateral ligament, with a corresponding increase in
ulnohumeral joint-opening consistent with the posterolateral rotatory
instability of the elbow seen clinically.
Clinical Relevance: The results of this study are consistent with
the clinical findings of posterolateral rotatory instability of the elbow
following long-standing cubitus varus and support the clinical concept that
increased strain in the lateral ulnar collateral ligament with this deformity
may eventually lead to overt posterolateral rotatory instability of the elbow.
We believe that these findings have implications for the etiology, prevention,
and treatment of posterolateral rotatory instability of the elbow following
cubitus varus deformity.