Ashutosh Khandha

Biomechanical rationale for using polyetheretherketone (PEEK) spacers for lumbar interbody fusion-A finite element study.

Study Design. To determine the effect of cage/spacer stiffness on the stresses in the bone graft and cage subsidence. Objective. To investigate the effect of cage stiffness on the biomechanics of the fused segment in the lumbar region using finite element analysis. Summary of Background Data. There are a wide variety of cage/spacer designs available for lumbar interbody fusion surgery. These range from circular, tapered, rectangular with and without curvature, and were initially manufactured using titanium alloy. Recent advances in the medical implant industry have resulted in using medical grade polyetheretherketone (PEEK). The biomechanical advantages of using different cage material in terms of stability, subsidence, and stresses in bone graft are not fully understood. Methods. A previously validated 3-dimensional, nonlinear finite element model of an intact L3–L5 segment was modified to simulate posterior interbody fusion spacers made of PEEK (“E” = 3.6 GPa) and titanium (“E” = 110 GPa) at the L4/5 disc with posterior instrumentation. Bone graft (“E” = 12 GPa) packed between the spacers in the intervertebral space was also simulated. The posterior lumbar interbody fusion spacer with instrumentation and graft represent a simulation of the condition present immediately after surgery. Results. The peak centroidal Von Mises stresses in the graft bone increased by at least 9-fold with PEEK spacers as compared to titanium spacer. The peak centroidal Von Mises stresses in the endplates increased by at least 2.4-fold with titanium spacers over the PEEK spacers. These stresses were concentrated at places where the spacer interfaced with the endplate. The stiffness of the spacer did not affect the relative motion (stability) across the instrumented (L4/5) segment. Conclusions. Spacers less stiff than the graft will: (1) provide stability similar to titanium cages in the presence of posterior instrumentation, (2) reduce the stresses in endplates adjacent to the spacers, and (3) increase the load transfer through the graft, as evident from the increase in stresses in graft.

Residual sagittal motion after lumbar fusion: a finite element analysis with implications on radiographic flexion-extension criteria.

Study Design. Finite element analysis of a lumbar fusion model. Objectives. To quantify residual sagittal angular motion following various types and levels of completeness of lumbar fusion in order to understand better the validity of current recommendations for interpreting flexion- extension radiographs to assess fusion. Summary of Background Data. Recommended threshold criteria for solid fusion using flexion-extension radiographs have varied from 0° to 5° of angular motion between vertebrae. Notwithstanding this wide variation and lack of uniform consensus, the validity of these criteria has not been previously biomechanically assessed to the authors’ knowledge. To investigate this issue, the authors sought to test various types of simulated healed, noninstrumented lumbar fusions using finite element modeling to determine the amount of residual angular motion under physiologic stresses. Methods. A validated 3-dimensional, nonlinear finite element model of an intact adult human L3–L4 motion segment was developed. Four fusion types were simulated using this model, including anterior lumbar interbody fusion (ALIF), posterior lumbar interbody fusion (PLIF), intertransverse process fusion, and interspinous process fusion. Variations of completeness of fusion were also represented. For ALIF and PLIF, this included tests of solid bridging bone within the posterior or anterior 75%, 50%, or 25% disc space. In addition, PLIF was also tested with either a unilateral or bilateral facetectomy to simulate commonly used surgical techniques. Variations of intertransverse process fusion included unilateral or bilateral bridging bone with or without medial fusion to the pars interarticularis. Only 1 scenario of a healed, solid interspinous process fusion was tested. The intact model and all fusion models were stressed with 10.6-Nm flexion and extension moments. The angular deflections were recorded in degrees. Results. A wide range of sagittal angular motion was recorded. For ALIF, this ranged from 0.8° (complete, 100% fusion) to 3.3° (solid fusion of the posterior 25% disc space). For PLIF, the numbers were more varied, ranging from 0.7° (complete, 100% fusion) to 6.9° (solid fusion of posterior 25% disc space with bilateral facetectomy). For intertransverse process fusion, the least motion was with a solid bilateral fusion, with medial healing to the pars (2.0°); the greatest motion was found with a solid unilateral fusion without medial healing (6.0°). Interspinous process fusion allowed only 1.9° of motion. Conclusions. The amount of residual flexion-extension motion with simulated lumbar fusions (presumably allowed by the bone’s inherent elasticity) under physiologically comparable moments varies with fusion type and, more substantially, with varying amounts of completeness. The current study documents a range of sagittal angular motion after several types of simulated lumbar fusion that appear to have considerable overlap with previously purported radiographic criteria for solid fusion using flexion-extension radiographs. However, it also suggests the possibility that some scenarios of solid, yet incomplete, fusion may allow motion that is substantially greater than 5°, which is beyond the most liberal of previously published threshold criteria.

Effect of lumbar interbody cage geometry on construct stability: a cadaveric study.

Study Design. Biomechanical study to investigate three-dimensional motion behavior of cadaveric spines in various surgical simulations. Objectives. To determine the effect of cage geometry on the construct stability. Summary of Background Data. There is a wide variety of cage/spacer designs available for lumbar interbody fusion surgery. These range from circular, tapered, and rectangular with and without curvature. However, the effectiveness of cages with different designs and materials to stabilize a decompressed intervertebral space has not been fully studied. Methods. Six fresh ligamentous lumbar spine specimens (L1–S2) were subjected to pure moments in the six loading directions. The resulting spatial orientations of the vertebrae were recorded using Optotrak™ Motion Measurement System. Measurements were made sequentially for intact, bilateral spacer placements across L4–L5 using a posterior approach, supplemented with pedicle screw-rod system fixation, and after the cyclic loading in flexion-extension mode. Results. The stability tended to decrease after the bilateral cage placement as compared with the intact for all loading cases except flexion. In flexion, the angular displacement decreased to 80% of the intact. However, there was no significant statistical difference seen in stability between intact and after bilateral spacer placement. Following the addition of posterior fixation using pedicle screw-rod system, the stability significantly increased in all directions. Cyclic loading did not have any significant effect on the stability. Conclusions. Stand-alone cages restore motion to near-intact levels at best, and supplement instrumentation is essential for significantly increasing the stability of the decompressed segment. The effects of cage geometry and Young’s modulus of the cage material do not seem to influence the stability, as compared with the other cagedesigns, especially after supplemental fixation with a posterior system.

Gait mechanics and second ACL rupture: Implications for delaying return-to-sport

Second anterior cruciate ligament rupture is a common and devastating injury among young women who return to sport after ACL reconstruction, but it is inadequately understood. The purpose of this study was to compare gait biomechanics and return‐to‐sport time frames in a matched cohort of young female athletes who, after primary ACLR, returned to sport without re‐injury or sustained a second ACL injury. Approximately 6 months after primary reconstruction, 14 young women (age 16 ± 2 years) involved in jumping, cutting, and pivoting sports underwent motion analysis testing after physical therapy and impairment resolution. Following objective return‐to‐sport clearance, seven athletes sustained a second ACL rupture within 20 months of surgery (13.4 ± 4.9 months). We matched them by age, sex, and sport‐level to seven athletes who returned to sports without re‐injury. Data were analyzed using a previously validated, EMG‐informed, patient‐specific musculoskeletal model. Compared to athletes without re‐injury, athletes who sustained a second ACL injury received surgery sooner (p = 0.023), had post‐operative impairments resolved earlier (p = 0.022), reached criterion‐based return‐to‐sport benchmarks earlier (p = 0.024), had higher body mass index (p = 0.039), and walked with lower peak knee flexor muscle forces bilaterally (p = 0.021). Athletes who sustained a second injury also tended to walk with larger (p = 0.089) and more symmetrical peak knee flexion angles and less co‐contraction, all indicative of a more normal gait pattern. Statement of Clinical Significance: Delayed return‐to‐sport clearance even in the absence of gait or clinical impairments following primary ACL reconstruction may be necessary to mitigate second ACL injury risk in young women.

Predictors of knee joint loading after anterior cruciate ligament reconstruction.

Anterior cruciate ligament (ACL) injury results in altered knee joint mechanics which frequently continue even after ACL reconstruction. The persistence of altered mechanical loading of the knee is of concern due to its likely role in the development of post‐traumatic osteoarthritis (OA). Joint contact forces are associated with post‐traumatic OA development, but evaluation of factors influencing the magnitude of contact forces after ACL injury is needed to advance current strategies aimed at preventing post‐traumatic OA. Therefore, the purpose of this study was to identify predictive factors of knee joint contact forces after ACL reconstruction. Thirty athletes completed standard gait analysis with surface electromyography 6 months after ACL reconstruction. An electromyographic‐driven musculoskeletal model was used to estimate joint contact forces. External knee adduction moment was a significant predictor of medial compartment contact forces in both limbs, while vertical ground reaction force and co‐contraction only contributed significantly in the uninvolved limb. The large influence of the knee adduction moment on joint contact forces provides mechanistic clues to understanding the mechanical pathway of post‐traumatic OA after ACL injury.

Gait mechanics in those with/without medial compartment knee osteoarthritis 5 years after anterior cruciate ligament reconstruction

The objective of the study was to evaluate differences in gait mechanics 5 years after unilateral anterior cruciate ligament reconstruction surgery, for non‐osteoarthritic (n = 24) versus osteoarthritic (n = 9) subjects. For the involved knee, the osteoarthritic group demonstrated significantly lower peak knee flexion angles (non‐osteoarthritic = 24.3 ± 4.6°, osteoarthritic = 19.1 ± 2.9°, p = 0.01) and peak knee flexion moments (non‐osteoarthritic = 5.3 ± 1.2% Body Weight × Height, osteoarthritic = 4.4 ± 1.2% Body Weight × Height, p = 0.05). Differences in peak knee adduction moment approached significance, with a higher magnitude for the osteoarthritic group (non‐osteoarthritic = 2.4 ± 0.8% Body Weight × Height, osteoarthritic = 2.9 ± 0.5% Body Weight × Height, p = 0.09). Peak medial compartment joint load was evaluated using electromyography‐informed neuromusculoskeletal modeling. Peak medial compartment joint load in the involved knee for the two groups was not different (non‐osteoarthritic = 2.4 ± 0.4 Body Weight, osteoarthritic = 2.3 ± 0.6 Body Weight). The results suggest that subjects with dissimilar peak knee moments can have similar peak medial compartment joint load magnitudes. There was no evidence of inter‐limb asymmetry for either group. Given the presence of inter‐group differences (non‐osteoarthritic vs. osteoarthritic) for the involved knee, but an absence of inter‐limb asymmetry in either group, it may be necessary to evaluate how symmetry is achieved, over time, and to differentiate between good versus bad inter‐limb symmetry, when evaluating knee gait parameters.

Gait mechanics and tibiofemoral loading in men of the ACL-SPORTS randomized control trial: ACL-SPORTS TRAINING AND GAIT MECHANICS

The risk for post‐traumatic osteoarthritis is elevated after anterior cruciate ligament reconstruction (ACLR), and may be especially high among individuals with aberrant walking mechanics, such as medial tibiofemoral joint underloading 6 months postoperatively. Rehabilitation training programs have been proposed as one strategy to address aberrant gait mechanics. We developed the anterior cruciate ligament specialized post‐operative return‐to‐sports (ACL‐SPORTS) randomized control trial to test the effect of 10 post‐operative training sessions consisting of strength, agility, plyometric, and secondary prevention exercises (SAPP) or SAPP plus perturbation (SAPP + PERT) training on gait mechanics after ACLR. A total of 40 male athletes (age 23 ± 7 years) after primary ACLR were randomized to SAPP or SAPP + PERT training and tested at three distinct, post‐operative time points: 1) after impairment resolution (Pre‐training); 2) following 10 training sessions (Post‐training); and 3) 2 years after ACLR. Knee kinematic and kinetic variables as well as muscle and joint contact forces were calculated via inverse dynamics and a validated electromyography‐informed musculoskeletal model. There were no significant improvements from Pre‐training to Post‐training in either intervention group. Smaller peak knee flexion angles, extension moments, extensor muscle forces, medial compartment contact forces, and tibiofemoral contact forces were present across group and time, however the magnitude of interlimb differences were generally smaller and likely not meaningful 2 years postoperatively. Neither SAPP nor SAPP + PERT training appears effective at altering gait mechanics in men in the short‐term; however, meaningful gait asymmetries mostly resolved between post‐training and 2 years after ACLR regardless of intervention group.

High muscle co-contraction does not result in high joint forces during gait in anterior cruciate ligament deficient knees: MUSCLE CO-CONTRACTION AND JOINT FORCES

The mechanism of knee osteoarthritis development after anterior cruciate ligament injuries is poorly understood. The objective of this study was to evaluate knee gait variables, muscle co‐contraction indices and knee joint loading in young subjects with anterior cruciate ligament deficiency (ACLD, n = 36), versus control subjects (n = 12). A validated, electromyography‐informed model was used to estimate joint loading. For the involved limb of ACLD subjects versus control, muscle co‐contraction indices were higher for the medial (p = 0.018, effect size = 0.93) and lateral (p = 0.028, effect size = 0.83) agonist–antagonist muscle pairs. Despite higher muscle co‐contraction, medial compartment contact force was lower for the involved limb, compared to both the uninvolved limb (mean difference = 0.39 body weight, p = 0.009, effect size = 0.70) as well as the control limb (mean difference = 0.57 body weight, p = 0.007, effect size = 1.14). Similar observations were made for total contact force. For involved versus uninvolved limb, the ACLD group demonstrated lower vertical ground reaction force (mean difference = 0.08 body weight, p = 0.010, effect size = 0.70) and knee flexion moment (mean difference = 1.32% body weight * height, p = 0.003, effect size = 0.76), during weight acceptance. These results indicate that high muscle co‐contraction does not always result in high knee joint loading, which is thought to be associated with knee osteoarthritis. Long‐term follow‐up is required to evaluate how gait alterations progress in non‐osteoarthritic versus osteoarthritic subjects.

Quantifying Motion Across a Solid Lumbar Interbody Fusion Using a Finite Element Model

Clinical assessment of pseudarthrosis or solid fusion is based on the residual motion across the “fused” segment (Kowalski et al, 2001). Dynamic flexion/extension (F/E) radiographs are commonly used to determine residual motion. Despite widespread use, it is unclear what the appropriate “cut-off” criteria to declare a fusion solid should be, with recommendations ranging from 0 to 5°. These values have not been derived by scientific methods. The present study was initiated to predict the angular sagittal motion across simulated lumbar interbody fusions (IF) using a Finite Element Model (FEM) of the ligamentous lumbar spinal segment. Anterior and posterior lumbar interbody fusions were simulated at the L3–L4 level as per the clinical procedure. Varying degrees of fusion were taken into account and the fusion mass was the simulated as a cancellous core with a cortical shell. The results indicated that 0.5° to 5.14° of angular motion can occur depending on fusion location and degree of completeness. While continuous bone might be noted at surgical exploration, this amount of motion may enable persistent loading of remaining structures, such as the annulus or spinal ligaments. In our view, this may prompt a redefinition of clinically “solid fusion”.Copyright