Effects of specimen length on the monosegmental motion behavior of the lumbar spine

Study Design. An in vitro biomechanical analysis of the segmental motion behavior of the same segments in polysegmental (five segments), bisegmental, and monosegmental specimens using sheep lumbosacral spines. Objectives. To investigate the effect of specimen length on monosegmental motion behavior. These data may be helpful in planning in vitro tests and in comparing results of studies using specimens of different lengths. Summary of Background Data. The length of spinal specimens used for in vitro stability tests varies greatly, depending on the purpose of the study. Some investigators prefer testing specimens with one adjacent segment on either end of the region of interest. Others favor specimens as short as possible. Methods. In a first step, seven sheep spine specimens, L3-S1 (note that sheep spines normally have seven lumbar vertebrae), each were tested without preload in a spine-loading apparatus. Alternating sequences of pure lateral bending, flexion/extension, and axial rotation moments (+/-3.75 Nm) were applied continuously. The motion in each single segment was measured simultaneously. Then, these polysegmental specimens were cut into two bisegmental specimens, L3-L5 and L6-S1, and tested in the same way. Finally, another vertebra was removed to obtain two monosegmental specimens, L3-L4 and L7-S1, and to test them as described. Results. In general, the range of motion at L3-L4 and L7-S1 was smaller when tested in polysegmental than in monosegmental specimens. In polysegmental specimens (five segments), the range of motion at L3-L4 and L7-S1 was approximately 80% (range, 70.6-92.5%) and in bisegmental specimens approximately 95% (range, 66.7-100%) of their range of motion measured in monosegmental specimens. Neutral zone and coupled motions showed the inverse behavior. Significant differences were found. However, they were not consistent with either the loading direction or with the specimen length. Conclusions. For comparison of results, the specimen length should be kept constant within one experiment. Segmental motion behavior of specimens with different lengths should be compared only qualitatively.