Intradiscal thermal therapy using interstitial ultrasound: an in vivo investigation in ovine cervical spine.

STUDY DESIGN: In vivo investigation of intradiscal ultrasound thermal therapy in ovine cervical spine model. OBJECTIVE: To evaluate the potential of interstitial ultrasound for selective heating of intradiscal tissue in vivo. SUMMARY OF BACKGROUND DATA: Application of heat in the spine using resistive wire and radiofrequency current heating devices is currently being used clinically for minimally invasive treatment of discogenic low back pain. Treatment temperatures are representative of those required for thermal necrosis of ingrowing nociceptor nerve fibers and disc cellularity alone, or with coagulation and restructuring of anular collagen in the high temperature case. METHODS: Two interstitial ultrasound applicator design configurations with directional heating patterns were evaluated in vivo in ovine cervical intervertebral discs (n = 62), with up to 45-day survival periods. Two heating protocols were employed in which the temperature measured 5 mm away from the applicator was controlled to either <54 C (capable of nerve and cellular necrosis) or >70 C (for coagulation of collagen) for a 10-minute treatment period. Transient and steady state temperature maps, calculated thermal doses (t43), and histology were used to assess the thermal treatments. RESULTS: These studies demonstrated the capability to control spatial temperature distributions within selected regions of the in vivo intervertebral disc and anular wall using interstitial ultrasound. CONCLUSIONS: Ultrasound energy is capable of penetrating within the highly attenuating disc tissue to produce more extensive radial thermal penetration, lower maximum intradiscal temperature, and shorter treatment times than can be achieved with current clinical intradiscal heating technology. Thus, interstitial ultrasound offers potential as a more precise and faster heating modality for the clinical management of low back pain and studies of thermal effects on disc tissue in animal models.

In vitro system for applying cyclic loads to connective tissues under displacement or force control.

Overuse is thought to be the primary cause of chronic tendon injuries, in which forceful or repetitive loading results in an accumulation of micro-tears leading to a maladaptive repair response. In vitro organ culture models provide a useful method for examining how specific loading patterns affect the cellular response to load which may explain the early mechanisms of tissue injury associated with tendinopathies and ligament injuries. We designed a novel tissue loading system which employs closed-loop force feedback, capable of loading six tissue samples independently under force or displacement control. The system was capable of applying loads up to 40 N at rates of 100 N s(-1) and frequencies of 2 Hz, well above loads and rates measured in rabbit tendons in vivo. Loading parameters such as amplitude, rate, and frequency can be controlled while biomechanical factors such as creep, force relaxation, tangent modulus and Young's modulus can be assessed. The system can be used to examine the relationship between each loading parameter and biomechanical factors of connective tissues maintained in culture which may provide useful information regarding the etiology of overuse injuries.

Basic fibroblast growth factor improves trabecular bone connectivity and bone strength in the lumbar vertebral body of osteopenic rats.

Recently, basic fibroblast growth factor (bFGF) has been found to increase trabecular bone mass and connectivity in the proximal tibial metaphyses (PTM) in osteopenic rats. The purpose of this study was to determine the bone anabolic effects of bFGF in the lumbar vertebral body (LVB), a less loaded skeletal site with a lower rate of bone turnover than the PTM. Six-month old female Sprague-Dawley rats were ovariectomized (OVX) or sham-operated and untreated for 8 weeks to induce osteopenia. Then group 1 (sham) and group 2 (OVX) were treated subcutaneously (s.c.) with vehicle, and OVXed groups 3 and 4 were treated s.c. with PTH [hPTH (1-34) at 40 microg/kg, 5x/week] and bFGF (1 mg/kg, 5x/week), respectively, for 8 weeks. At sacrifice, the fifth LVB was removed, subjected to micro-CT for determination of trabecular bone structure and then processed for histomorphometry to assess bone turnover. The sixth LVB was used for mechanical compression testing (MTS, Bionix 858). The data were analyzed with the Kruskal-Wallis test followed by post-hoc testing as needed. After 16 weeks of estrogen deficiency, there were significant reductions in vertebral trabecular bone volume and trabecular thickness. Treatment with either bFGF or hPTH (1-34) increased BV/TV in OVX animals. Human PTH (1-34)-treated animals had significant increases in trabecular (48%) and cortical thickness (30%) and bone strength [maximum load (53%) and work to failure (175%)] compared to OVX + Vehicle animals. Treatment of osteopenic rats with bFGF increased bone volume (15%), trabecular thickness (13%), maximum load (45%) and work to failure (140%) compared to OVX + Vehicle animals (all P <0.05). Basic FGF increased trabecular bone volume in the lumbar vertebral body of osteopenic rats by restoring trabecular number, thickness and connectivity density. Also, bFGF improved bone mechanical properties (maximum force and work to failure) compared to the OVX + Vehicle group. Therefore, increasing the number, thickness and connections of the trabeculae contributes to increased bone strength in this small animal model of osteoporosis.

A three-dimensional MRI analysis of knee kinematics.

PURPOSE: To quantify normal, in vivo tibio-femoral knee joint kinematics in multiple weight bearing positions using non-invasive, high-resolution MRI and discuss the potential of developing future kinematic methods to assess patients with abnormal joint pathologies. METHODS: Ten volunteers with clinically normal knees pushed inferiorly on the footplate of a weight bearing apparatus inside the MR scanner. The volunteers held the weight (133 N) for five scans as the knee motion was evaluated from 0 degrees to 60 degrees of flexion. Full extension was set as the zero point for all measured parameters. Using 3D reconstructions, tibia motion relative to the femur and flexion angle was measured as varus-valgus angle, axial rotation, anterior-posterior translation, and medial-lateral translation. Medial and lateral compartment tibio-femoral contact areas were examined and centroids of the contract areas were calculated. RESULTS: Tibial internal rotation averaged 4.8 degrees at 40 degrees of flexion and then decreased. Tibial valgus increased by 8 degrees at 60 degrees of flexion. Femoral roll back also increased to 18.5 mm average at 60 degrees of flexion, while the tibia translated medially 2.5 mm. Medial compartment femoro-tibial contact area started at 374 mm2 and decreased to 308 mm2 with flexion of 60 degrees, while lateral compartment contact area did not change significantly from 276 mm2. CONCLUSIONS: Results correlate with previous studies of knee kinematics while providing greater three-dimensional detail. MR imaging allows excellent non-invasive evaluation of knee joint kinematics with weight bearing. This tool may potentially be used for assessing knee kinematics in patients with knee pathology.

Impact biomechanics and pelvic deformation during insertion of press-fit acetabular cups.

Five fresh cadaver pelves were cleaned of soft tissue and instrumented with strain gauges. The acetabula were reamed, and a cementless cup, oversized at the periphery, was inserted. The applied force and cup acceleration were measured during insertion and used to calculate an effective mass of the cup, insertion device, and pelvis during each impact. Periacetabular strains increased variably during cup seating. After the cups were seated, strains continued to increase with postseating impacts. The effective mass remained constant throughout the test, indicating that cup seating is not associated with a change in acceleration. This finding implies that an accurate assessment of cup seating cannot be inferred by surgeon proprioception during impaction, and use of an apical hole in the cup is necessary to determine when the cup has seated.