Finite element model of spinal hemiepiphysiodesis: effect of contact conditions, initial conditions, and growth

Growth modification is under investigation to treat pediatric spine deformities. A hemiepiphyseal staple construct has been shown to alter growth and create physeal structural gradients in an in vivo porcine model. A finite element model (FEM) of a motion segment with and without implant was developed based on preliminary experimental results of initial post-operative motion segment compressive properties. The nonlinear tangent stiffness determined from the model correlated well with the experiments for the native segment, whereas after addition of the implant, the model overestimated the stiffness. The current purpose was to determine the effect of implant-bone contact conditions and initial disc displacement conditions, include growth effects, and compare FEM and experimental results at each stage. A 3D FEM was developed from a CT scan of a porcine T7-T8 segment. The annulus was modeled as an incompressible anisotropic hyperelastic material, and the nucleus as an incompressible fluid. A CAD model of the implant was constructed. Load-displacement curves in compression were determined from a nonlinear analysis performed under different initial and bone-implant interface conditions. Contact conditions were a) perfect, b) friction of 0.1-0.3, or c) soft contact. Initial conditions were that implant insertion induced a) no change in stress or strain in the disc, b) a 2˚ angulation with a centrally located neutral axis and no residual stresses, and c) both stress and coronal plane displacement gradients. Growth modulation effects were added using a published linear relationship between compressive stress and growth rate. A 2 month PO time was simulated. Altering bone - implant surface contact conditions from perfect to either friction or soft contact decreased the stiffness, but all models remained stiffer than experimental results. An initial disc angulation without residual stress did not affect stiffness, whereas stiffness increased with an initial angle and compressive stress. The growth simulation results predicted 32% of control growth on the side ipsilateral to the implant and 81% on the contralateral side. Reductions in growth were similar in pattern, but overestimated, experimental histomorphometric changes.

Spinal hemiepiphysiodesis correlates with physeal histomorphometric gradients

Compression of a growth plate is known to correlate with changes to growth plate structure. The purpose of this study was to determine if the height of the physeal hypertrophic zones and chondrocyte size were directly related to the distance from a spine implant. The implant was designed with the goal of slowing spine growth asymmetrically. For stapled vertebrae, hypertrophic zone height, cell height and cell width were decreased across the coronal plane of the growth plate, with the lowest values under the staple, 8 weeks postoperatively.

Bilateral intra-annular spinal compressive stresses in vivo

The purpose of this pilot study was to determine if compressive stresses in the annulus of the intervertebral disc vary with activity in a quadruped and are affected by treatment with an implant. Pilot in vivo tests were conducted on skeletally immature domestic pigs (approved by IACUC). One pair of sensors was implanted within the annulus of T10-11, and the second pair at T8-9. A staple was them implanted across the right side of T8-9. Wires were routed subcutaneously and exited at the dorsal cervical region. Sensor signals were acquired before and after staple implantation, post-operatively during normal activities, and biweekly under anesthesia. After 8 weeks, spines were harvested and imaged. Early results from 2 sensors during walking and sitting, post-op day 5, clearly showed cyclic stresses during gait. Stresses were attenuated at the stapled vertebra compared to the unstapled vertebra.

Growth plate forces in the adolescent human knee: a radiographic and mechanical study of epiphyseal staples

The purpose of this study was to quantify the forces produced by the growth plates of the adolescent human proximal tibia and distal femur. The postoperative deformations of staples used to treat genu valgum were correlated with similar staple deformations produced by mechanical testing. Staple deformation was measured in sequential radiographs of 35 knees in 20 patients. Mechanical tests were performed on new staples to determine the force necessary to bend the staples to the observed displacements. Based on deformation at final examination, the static equivalent loads exerted at the joint centerline by the growth plates of the proximal tibia and distal femur were 0.5 kN per physis. The estimated corresponding stress was 1 MPa. These results indicated that successful lower limb hemiepiphysiodesis suppressed a longitudinally directed, centrally applied compressive force on the order of body weight.

Endoscopic discectomy increases thoracic spine flexibility as effectively as open discectomy. A mechanical study in a porcine model

STUDY DESIGN

Two surgical techniques for anterior discectomy were compared biomechanically. The surgical procedures were performed in live, anesthetized, skeletally immature pigs. Spine flexibility was measured in vitro.

OBJECTIVE

To determine whether endoscopic techniques for discectomy are as effective as open procedures in increasing spine flexibility.

SUMMARY OF BACKGROUND DATA

Although studies have verified that discectomy increases spine flexibility, no study has confirmed whether endoscopic techniques increase flexibility as effectively as standard thoracotomy, which is a substantially different procedure.

METHODS

The intervertebral disc between vertebrae T8 and T9 was resected from 30 live, anesthetized, adolescent pigs. In 15 pigs, the chest was opened via thoracotomy of the eighth rib, and the disc was excised. In the other 15 pigs, the disc was removed endoscopically. These motion segments and six intact controls were tested mechanically in side bending, flexion-extension, and axial rotation.

RESULTS

No statistically significant differences in flexibility were found between open and endoscopic groups in any loading direction. The statistical power to detect a 20% difference between surgical groups was > or = 95%.

CONCLUSIONS

Endoscopic and open techniques were equally effective in increasing spine flexibility. Because endoscopy may reduce surgical morbidity compared with open discectomy, these results support the use of endoscopy for the surgical correction of scoliosis before instrumentation.

Effect of tibial attachment location on the healing of the anterior cruciate ligament freeze model

We studied the healing response of a devitalized anterior cruciate ligament to a treatment of initial anterior-posterior joint translation in goats. Devitalization and devascularization were achieved by five successive freeze-thaw cycles. Anterior-posterior translation was surgically altered by an osteotomy of the tibial attachment of the devitalized ligament and its reattachment either in the anatomical position or in a position 5 mm posterior. Six weeks after the first surgery, the same procedure was performed on the contralateral limb, except that the ligament was reattached in the alternate position. Six months after the initial surgery, femur-anterior cruciate ligament-tibia specimens were tested to determine their structural and mechanical material properties. Anatomic ligament placement resulted in reduced anterior-posterior translation (p < 0.05) and greater anterior joint stiffness (p < 0.05). Maximum load (p < 0.05) and ligament stiffness (p < 0.01) also were greater for the anatomically placed anterior cruciate ligaments. The maximum load for anatomically placed ligaments averaged 1.625 +/- 211 N (SEM). The strength of the posteriorly placed anterior cruciate ligament, 895 +/- 164 N was similar to results of historical anterior cruciate autograft reconstructions. Ligament failure occurred near the tibial insertion in the posteriorly placed ligaments more often than in the anatomically placed ligaments (four of five times compared with one of five times). Ligament failure near the tibial insertion occurred with lower mean maximum load than failure at the midsubstance or by bone avulsion (796 compared with 1.592 N: p < 0.05). These data support the hypothesis that ligament laxity is important to the healing and remodeling of anterior cruciate ligament grafts.

In vivo forces in the anterior cruciate ligament: direct measurements during walking and trotting in a quadruped

In vivo forces in the anterior cruciate ligament (ACL) were measured in three adult goats during quiet standing and during gait (walking or trotting). A modified pressure transducer (MPT) was implanted within the anteromedial band of the ligament to make direct measurements of ACL force. One or two days following implantation, measurements were made of ACL force, knee joint flexion angle, ground reaction forces, and speed of locomotion. MPT calibration was performed in vitro using anteroposterior displacement tests at six flexion angles. The ACL was loaded during quiet standing (30-61 N) and during the stance phase of gait. Peak ACL forces were achieved within the first 40% of stance, with magnitudes ranging from 63 to 124 N during walking and from 102 to 150 N during trotting. The average ACL force during the stance phase ranged from 34 to 68 N while walking and from 46 to 69 N while trotting. The partial correlations between peak ACL force and speed, and between average ACL force and speed, were both statistically different from zero (p < 0.01). ACL forces dropped to zero during the swing phase in all trials. ACL forces were less than 15 N throughout swing in two of the animals, both of which did not extend their knees during gait beyond 20 degrees from full extension. In the animal which did show knee extension beyond 40 degrees (20 degrees from full extension), ACL loading occurred during late swing. The magnitude of the peak ACL force during late swing was significantly correlated with the extent of knee extension in this animal.

Displacements of the menisci under joint load: an in vitro study in human knees

The purpose of this study was to test whether the menisci displace under joint compression combined with internal-external torques and anterior-posterior forces at fixed flexion angles. We further determined differences in displacements between the medial and lateral menisci. Loads were applied to the joint, and joint load and displacements were measured. Meniscal displacements were measured radiographically. With a joint compressive load of 1 kN, internal and external joint rotations caused the lateral meniscus to displace, on average, 0.37 mm deg-1 in the anterior-posterior direction, while the medial meniscus displaced 0.19 mm deg-1. Anterior and posterior joint translation, performed under 1 kN joint compression, caused the lateral meniscus to displace, on average, 0.66 mm mm-1 in the anterior-posterior direction, while the medial meniscus displaced 0.43 mm mm-1. Greater meniscal displacements were found at 15 and 30 degrees flexion than at 0 degrees for the lateral meniscus in internal rotation. Lateral meniscal displacements were larger than those of the medial with posterior tibial translation at full extension and with internal rotation.

Knee joint contact pressure decreases after chronic meniscectomy relative to the acutely meniscectomized joint: a mechanical study in the goat

Several studies have shown that meniscectomy causes an immediate, acute increase in knee joint contact pressure and that changes in pressure distribution cause remodeling of bone and soft tissue. Presumably, this remodeling in turn affects contact pressures. This study tested the hypothesis that medial compartment contact pressure increases immediately after medial meniscectomy and then decreases with time. Supporting hypotheses regarding medial compartment contact area and lateral compartment pressures also were tested. Unilateral medial meniscectomy was performed on seven adult goats. Four or 8 months later, contact pressure and area were measured in vitro in the involved joints, as well as in the contralateral joints, before and after removal of the meniscus. The medial compartment pressures of the chronically meniscectomized joints were significantly less than those of the acutely meniscectomized paired joints but remained significantly greater than those of the intact joints. For the 4 and 8 month groups combined, the mean pressures of the acutely and chronically meniscectomized joints were greater than the pressures of the paired intact joints by 70 and 42%, respectively. The mean medial compartment contact areas of the acute and chronic joints were lower than those of the intact joints by 60 and 50%, respectively; mean lateral compartment pressures remained the same. This study indicates that joint remodeling reduces joint contact pressures. It also suggests that the effectiveness of a treatment to reduce pressure concentrations may be determined only by comparison, at the same postoperative time, of the pressure with that of the chronically meniscectomized joint, since pressures decreased with time without treatment.

Effects of gamma irradiation on the initial mechanical and material properties of goat bone-patellar tendon-bone allografts

The effects of 60Co gamma irradiation on the initial mechanical properties of the composite bone-patellar tendon-bone unit (CU) and the tendon midsubstance (TM) were studied. Frozen specimens were exposed to either 2 or 3 Mrad of gamma irradiation. Paired frozen specimens served as intraanimal controls. Treatment effects on the CU were assessed using four mechanical parameters. Effects on the TM were assessed using four material parameters measured using an optical surface-strain analysis system. The maximum force and strain energy to maximum force of the composite unit were significantly reduced 27% and 40%, respectively, after 3 Mrad of irradiation (p less than .05). Mechanical properties of the CU were not significantly altered, however, following 2 Mrad of irradiation. Based on individual paired contrasts between treatment and control, significant differences were also found in the material properties of the tendon midsubstance. The maximum stress, maximum strain, and strain energy density to maximum stress were significantly reduced following 3 Mrad, but not 2 Mrad, of irradiation. The results provide important "time zero" material property data, which will be useful for later anterior cruciate ligament reconstruction studies using irradiated allograft patellar tendons in the goat model and other animal models as well.

Anterior cruciate ligament replacements: a mechanical study of femoral attachment location, flexion angle at tensioning, and initial tension

We examined three surgical variables that affect the ability of an anterior cruciate ligament replacement to restore the limit of anterior tibial translation. These were the placement site of the substitute on the femur, the initial tension applied to the replacement, and the flexion angle of the knee at the time of tensioning. An anterior load of 100 N was applied to the tibia. As the knee was flexed, we measured the tensile force in the substitute and the anteroposterior position of the femur relative to the tibia. Placement largely determined whether the force in the replacement increased or decreased with flexion. Placement also largely determined whether the tibia moved anteriorly or posteriorly with flexion compared to its position in the intact knee. The initial tension and the flexion angle at tensioning affected the magnitude of force in the substitute and the magnitude of the change in AP position. They did not affect how force and AP position changed with flexion. Greater increases in force and greater posterior shifts in tibial position were produced by changing the flexion angle at tensioning from 0 degrees to 30 degrees than by increasing the initial tension from 22 to 44 N.