Study Design. Biomechanical laboratory research.
Objective. To characterize the structural stiffness of opening and closing wedge osteotomies and the independent effect of rod diameter.
Summary of Background Data. Traditionally, C7 opening wedge osteotomy (OWO) has been performed for patients with ankylosing spondylitis. For patients without ankylosing spondylitis, closing wedge osteotomy (CWO) may be considered for more controlled closure. Biomechanical characteristics of the two osteotomy alternatives have not yet been analyzed.
Methods. Nondestructive pure moment flexion/extension (FE), lateral bending (LB), and axial rotation (AR) tests were conducted to 4.5 Nm on cadaveric specimens (C4–T3). All specimens underwent posterior bilateral screw-rod fixation with 3.5 mm and 4.5 mm Ti rods, whereas half received OWO and half received CWO.
Results. Independent of osteotomy type, constructs with 4.5 mm rods exhibited a significant increase in stiffness compared to 3.5 mm rods in all bending modes (P < 0.01). Relative to 3.5 mm rods, 4.5 mm constructs showed an increase in stiffness of 31 ± 12% for FE, 37 ± 39% for LB, and 31 ± 11% for AR. At the osteotomy site, there was a 43 ± 23% increase in FE stiffness, 45 ± 36% in LB, and 41 ± 17% in AR. Independent of rod diameter, CWO was significantly stiffer than OWO (42% for the construct and 56% across the osteotomy) in FE bending only (P < 0.05).
Conclusion. The surgeon can expect a similar increase in stiffness in switching from 3.5 mm to 4.5 mm rod independent of osteotomy type. The increased stiffness of CWOs has an anatomic basis. OWOs disrupt the anterior longitudinal ligament (ALL) and leave a significant anterior gap whereas CWOs create a wedge through the vertebral body and leave the ALL and the discs above and below the osteotomy intact. The closure in CWOs leaves no anterior gap providing greater axial loading stability. This greater bone on bone contact in CWOs is likely a significant reason for the anterior stiffness and may provide greater fusion rates in the nonankylosing spondylitis patient population.
Pure moment biomechanical testing across the cervicothoracic junction showed that a closing wedge osteotomy is more mechanically stiff than an opening wedge osteotomy and increasing rod diameter from 3.5 mm to 4.5 mm results in a significant increase in rigidity.
*Biomechanical Testing Facility, Orthopaedic Trauma Institute, San Francisco General Hospital, San Francisco, CA
†Department of Orthopaedic Surgery, University of California, San Francisco, CA
‡Department of Neurological Surgery, University of California, San Francisco, CA
Address correspondence and reprint requests to Justin K. Scheer, BS, Biomechanical Testing Facility, San Francisco General Hospital, 1001 Potrero Ave, Building 9, 3rd Floor, San Francisco, CA 94110; E-mail: Justin.Scheer@gmail.com
Acknowledgment date: March 5, 2010. Revision date: April 28, 2010. Acceptance date: July 21, 2010.
The manuscript submitted does not contain information about medical device(s)/drug(s).
Research funding and implants were provided by Ulrich Medical. Statistical analysis conducted with partial support from NIH/NCRR UCSF-CTSI UL1 RR024131.