A biomechanical comparison of calf versus cadaver lumbar spine models.

STUDY DESIGN: Biomechanical flexibility tests were performed using calf and human cadaveric lumbar spine models to investigate the effect of anatomic differences. OBJECTIVES: The purpose is to determine if differences exist in biomechanical flexibility testing results between calf and human cadaveric spines when using identical methods and instrumentation. SUMMARY OF BACKGROUND DATA: Calf spines are commonly used in biomechanical research as a substitute for human cadaveric spines in an attempt to reduce expense and specimen variability. Despite widespread use, the validity of this model has not been thoroughly investigated. METHODS: Five fresh calf spines and five human cadaveric spines (L2-L5) were used for nondestructive biomechanical flexibility testing. Maximum moments of 6.4 Nm were achieved in five increments of 1.6 Nm. The rotations of L3 with respect to L4 were measured in 5 cases: 1) intact; 2) following partial discectomy, including partial laminectomy and partial facetectomy; 3) partial discectomy with pedicle screw instrumentation; 4) total discectomy with pedicle screw instrumentation; and 5) pedicle screw instrumentation with interbody graft. Rotational angles were normalized to the intact case to determine the stabilizing effect during each testing case. Data were analyzed using analysis of variance to determine if significant differences existed between the calf spine results and the human cadaveric spine results. RESULTS: In both models, motion increased following discectomy, decreased with instrumentation, and increased with total discectomy. Placement of the interbody graft decreased motion during axial rotation, flexion, and extension but increased lateral bending motion. A two-way analysis of variance revealed no significant differences in the two models during flexion or extension (P > 0.05), but significant differences were discovered in axial rotation and lateral bending (P < 0.05). CONCLUSIONS: Significant differences were identified in flexibility testing between calf and human cadaveric specimens. The calf spine model overestimated the stabilizing effect of instrumentation during lateral bending and underestimated stability during axial rotation. The extrapolation of calf spine data to the in vivo case, especially during axial rotation and lateral bending, should carefully consider the variation between these two models.