Biomechanical analysis of osteotomy type and rod diameter for treatment of cervicothoracic kyphosis.
METHODS
Biomechanical laboratory research.


OBJECTIVE
To characterize the structural stiffness of opening and closing wedge osteotomies and the independent effect of rod diameter.


BACKGROUND
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).


CONCLUSIONS
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.
Biomechanical Testing Facility, Orthopaedic Trauma Institute, San Francisco General Hospital, San Francisco, CA, USA. Justin.Scheer@gmail.com
Justin K
JK
Scheer
Jessica A
JA
Tang
Jenni M
JM
Buckley
Vedat
V
Deviren
Murat
M
Pekmezci
R Trigg
RT
McClellan
Christopher P
CP
Ames
D000368:Aged; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena; D001858:Bone Nails; D002102:Cadaver; D002574:Cervical Vertebrae / Q000473:pathology / Q000503:physiopathology / Q000601:surgery*; D005260:Female; D006801:Humans; D007738:Kyphosis / Q000503:physiopathology / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D010027:Osteotomy / Q000295:instrumentation / Q000379:methods*; D016059:Range of Motion, Articular; D012399:Rotation; D013904:Thoracic Vertebrae / Q000473:pathology / Q000503:physiopathology / Q000601:surgery*; D016896:Treatment Outcome