To The Editor:
In their very detailed study, "The Effect of the Orientation
of the Acetabular and Femoral Components on the Range of Motion
of the Hip at Different Head-Neck Ratios" (82-A: 315-321, March
2000), D'Lima et al. have missed one important point.
The introduction of high-nitrogen stainless steel and the cold
forming process has resulted in a much stronger Charnley stem, the
Ortron (Johnson and Johnson, Leeds, United Kingdom). The diameter
of the neck is reduced from 12.5 millimeters to ten millimeters
without loss of strength. Use of this stem has been standard in my
clinical practice since 1984.
One other point: true adduction of a natural hip is limited by
the contralateral thigh as some degree of flexion invariably takes
place.
B. M. Wroblewski, F.R.C.S.
Centre for Hip Surgery
Wrightington Hospital, Hall Lane, Appley Bridge
Wigan, Lancashire WN6 9EP, England
D. D. D'Lima, A. G. Urquhart, K. O. Buehler, R. H. Walker,
and C. W. Colwell, Jr., reply:
We greatly appreciate Mr. Wroblewski's comments on our study.
We are aware of several designs that have neck diameters smaller
than 12.25 millimeters and, therefore, larger head-neck ratios.
We also currently utilize narrow-neck-design components in our practice.
It is the head-neck ratio, not the absolute neck diameter, that
determines range of motion. The computer model that we described
can simulate any reasonable head-neck ratio. We agree that, for
a given set of variables, reducing the neck diameter will increase
the range of motion (as was demonstrated in the Results section
of our paper). This will shift the range-of-motion curves upward
without changing their shape. We hope that our simple model will
help to elucidate the complex interplay of component position and head-neck
ratio.
We also agree that adduction clinically involves some flexion
to clear the contralateral lower extremity. Again, this is not difficult
to simulate with the model that we described. We decided to measure
pure adduction, as it is defined kinematically, because we felt
that adduction near full hip extension was less relevant than adduction
at other hip flexion angles. We first studied the effect of prosthetic
orientation on each uncoupled range of motion (such as pure abduction,
pure adduction, and pure rotation); then we combined several variables
such as flexion, adduction, and rotation to simulate clinically
relevant positions or activities of daily living.
We hope that we have adequately addressed Mr. Wroblewski's concerns
and look forward to any correspondence in the future.
Darryl D. D'Lima, M.D.
Andrew G. Urquhart, M.D.
Knute O. Buehler, M.D.
Richard H. Walker, M.D.
Clifford W. Colwell, Jr., M.D.
Corresponding author: Clifford W. Colwell, Jr., M.D.
Division of Orthopaedic Surgery, Scripps Clinic
10666 North Torrey Pines Road, La Jolla, California 92037
E-mail address: colwell@scripps.edu