Mature runt cow lumbar intradiscal pressures and motion segment biomechanics

Abstract

Background context The optimal animal model for in vivo testing of spinal implants, particularly total or partial disc replacement devices, has not yet been determined. Mechanical and morphological similarities of calf and human spines have been reported; however, limitations of the calf model include open growth plates and oversized vertebrae with growth. Mature runt cows (Corrientes breed) may avoid these limitations. Purpose This study compared vertebral morphology and biomechanical properties of human and runt cow lumbar motion segments. Study design In vivo disc pressure measurements were obtained in six mature runt cows at L4–L5. In vitro evaluation was performed on these same segments and repeated on 12 human motion segments. Methods Disc pressures were measured in vivo in runt cow (Corrientes breed) L45 discs using a percutaneous transducer with the animal performing various activities. These motion segments were then harvested and morphologic and biomechanical evaluations (disc pressure in compression, flexibility tests to 7.5 Nm) were performed on both cow and male human L23 and L45 segments. Results The transverse lumbar disc dimensions were slightly smaller for (mixed gender) cow versus (male) humans, but were within the range of reported (mixed gender) human values. The mean±SD disc height was smaller for runt cow (7±1 mm) versus human discs (13±2 mm, p<.001). The vertebral bodies of the cow were approximately twice as tall as the human. In vitro testing revealed significantly greater disc pressure response to applied axial loading in the runt cow versus humans (1.27±0.18 vs. 0.84±0.15 kPa/N, respectively) but similar overall stiffness (2.15±0.71 vs. 1.91±0.94 kN/mm, respectively). Runt cow and human segment flexibility curves were similar with the following exceptions: runt cow stiffness was ∼40% greater in torsion (p<.05), runt cow segment lateral bending motion was greater versus humans (range of motion by 30%, neutral zone by 100%; both p<.05), and flexion range of motion tended to be smaller in runt cow versus human specimens (by ∼40%, p=NS). In vivo, the standing disc pressure in the runt cow was 0.80±0.24 MPa. Conclusions Although no animal replicates the human motion segment, the runt cow lumbar spine had a number of biomechanical and morphological measurements within the range of human values. The closed physes and temporally stable morphology of the mature runt cow may make this model more suitable versus standard calf models for human intradiscal implant studies.