Effect of increasing implant height on lumbar spine kinematics and foraminal size using the ProDisc-L prosthesis.
METHODS
A biomechanical study using human lumbar spines.


OBJECTIVE
To test the hypotheses that with increasing implant height (1) the range of motion (ROM) of the implanted segment will decrease, (2) the segmental lordosis will increase, and (3) the size of the neural foramens will increase.


BACKGROUND
Little is known about the effects of the implant height on the segmental motion and foraminal size at the implanted level.


METHODS
Seven human lumbar spines (age, 54.4+/-11.4 years; L1-sacrum) were tested intact, and after discectomy at L4-L5 and sequential insertion of ProDisc-L implants (Synthes Spine, Paoli, PA) of increasing heights (10, 12, and 14 mm). The specimens were tested in flexion (8 Nm) and extension (-6 Nm) with a 400 N follower preload as well as in lateral bending (+/-6 Nm) and axial rotation (+/-5 Nm) without preload. Three-dimensional motions were measured at L4-L5. Foraminal sizes at L4-L5 were measured in the specimen's neutral posture under a 400 N preload for the intact spine and after each implantation using finely graded cylindrical probes. Segmental lordosis was measured in the specimen's neutral posture under a 400 N preload by analyzing digital fluoroscopic images. Effects of implant height on the kinematics, foraminal size, and segmental lordosis were assessed using paired comparisons with Bonferroni correction.


RESULTS
Increasing implant height from 10 mm to 14 mm caused a significant decrease (P<0.05) in segmental ROM by up to 37%+/-21% in flexion/extension, 33%+/-18% in lateral bending, and 29%+/-28% in axial rotation. Increasing implant height also produced a significant increase in segmental lordosis (P<0.05): from 9.7 degrees+/-2.9 degrees at 10 mm, to 16.1 degrees+/-5.1 degrees at 14 mm. The increase in foraminal size, while significant, was only 4.6%+/-3.2% when comparing 10 mm to 14 mm implants.


CONCLUSIONS
These results suggest that a smaller implant height should be selected to optimize the ROM of the implanted segment and maintain sagittal balance.
Gaffey
John L
JL
Department of Orthopaedic Surgery and Rehabilitation, Loyola University Medical Center, Maywood, IL 60153, USA.
Ghanayem
Alexander J
AJ
Voronov
Michael L
ML
Havey
Robert M
RM
Carandang
Gerard
G
Abjornson
Celeste
C
Patwardhan
Avinash G
AG
D000328:Adult; D001178:Arthroplasty / Q000295:instrumentation*; D001696:Biomechanical Phenomena; D017586:Diskectomy; D005260:Female; D006801:Humans; D007403:Intervertebral Disc / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000601:surgery*; D008141:Lordosis / Q000503:physiopathology; D008159:Lumbar Vertebrae / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D004856:Postural Balance; D011474:Prosthesis Design; D011859:Radiography; D016059:Range of Motion, Articular; D016474:Weight-Bearing