In vivo dynamic compressive stresses in the disc annulus: a pilot study of bilateral differences due to hemiepiphyseal implant in a quadruped model.

In vivo biomechanical study in quadruped model. To develop in vivo model capable of determining physiological compressive stresses bilaterally in the intervertebral disc annulus and preliminarily assess effects of a hemiepiphyseal implant. Spine growth modification alters stress distributions in vertebral growth plates and discs. Quantification of stresses is required to help assess implant efficacy and disc health. More generally, despite widespread and necessary use of animals in preclinical studies of spine instrumentation, limited quantitative information is available on mechanobiological conditions in quadruped spines for comparisons with those of humans. Skeletally immature domestic pigs were instrumented with an implant and 4 stress sensors. Sensors were inserted into left and right sides of the annulus at 2 thoracic levels. A titanium staple-screw construct was implanted at 1 level. Signals were acquired intraoperatively, postoperatively during normal activities, and biweekly with the animal under anesthesia, for up to 8 weeks. Stresses varied by sensor location relative to implant, postoperative time, activity, and animal. Intraoperatively, the mean peak stress due to staple insertion was 1.6 MPa at the sensor nearest the staple. Mean stress at the end of surgery was 0.23 MPa. Mean stress standing the first day was 0.38 MPa. Dynamic stresses were recorded at all locations, including the location nearest the staple. Highest mean stresses were those nearest the implant. With the animal under anesthesia, the dynamic stress range in the resting prone position was 0.1 MPa, whereas this range was 0.9 MPa when the spine was manually flexed. Compressive stresses were dynamic at both control and stapled levels, which indicated that the disc was not immobilized by the implant. These pilot results suggested that mean disc compression was increased within the first postoperative week. Stresses ranged up to levels measured in humans.