Biomechanics Research Projects

In osteoporotic vertebral bodies a significant increase in strength of fixation and reduction of loosening under cyclic load with pedicle screws was achieved with a small amount of bone cement injected through the screw and into the threads.*

Pedicle screw anchoring was compared to cortical screw anchoring in cadaver lumbar vertebral bodies after a regimen of severe cyclic loading. The cortical screws had stronger fixation in the bones of normal vertebral bodies, but pedicle screw fixation was superior to cortical screws in osteoporotic vertebral bodies.

With minimal invasive surgical techniques, the anterior support in lumbar constructs must use smaller cages and anchoring systems. A series of studies is designed to address the issues of where is the best location to place small cages to optimize the biomechanics of the construct, and to investigate the potential of cages that expand.

Sawbones has developed a mechanical model for testing constructs in the lumbar spine. Over 30 years of testing constructs in the lumbar spine at MBI, we accumulated a series of intact 39 lumbar spines tested with the same loading and measurements systems prior to any surgical alterations. We tested the Sawbones spine exactly the same as our cadaver spines. The purpose: validate the Sawbones spine model.

Loading of cadaver lumbar spines through flexion-extension range of motion was monitored by fluoroscopic videos. The effects of several surgical procedures on each motion segment unit was recorded and used to verify a computer model of the lumbar spine for each of those procedures. The goal is to create a patient specific computer model that can simulate selected surgical procedures as a surgical planning tool for spine surgeons. (Note: the model won a 1st place award for spine research when presented at the annual Ortho Research Soc. Meeting) 

Cadaver thoraco-lumbar spines were loaded in flexion-extension before and after lateral access anterior instrumentation with and without posterior support.

In cadaver lumbar spines mapping of trabecular bone hydraulic permeability and elastic modulus correlated with bone microstructural parameters measured via micro-CT. Comparison of normal and osteoporotic showed potential for predicting vulnerability to collapse in osteoporotic vertebral bodies by computer modelling of the microstructure from CT scans. The goal is to identify vulnerable patients and proactively begin treatments to prevent collapse in osteoporotic patients. 

Proximal junctional kyphosis can occur at the proximal end of long constructs in adult scoliosis. The aim is to evaluate various strategies to lessen the sudden change in rigidity and load concentration at the end of the constructs. A pilot study in thoracic cadaver spines is underway to see what loading conditions and spinal constructs will create similar mechanical failures to those seen clinically. Once the model is established, multiple hypotheses can be tested to reduce the chance of clinical failures.

Anterior cage and plate fixation after 3 level discectomy was compared to stand alone cage fixation in cadaver cervical spines. Minor bone resorption was simulated by placing a thin, 0.2 mm shim in between the cage and the vertebral body end plates at each level. After recording the cage-end plate pressure maps through a range of motion in flexion-extension, the shim was removed and the recordings repeated. The pressures were significantly lowered in the plated constructs with increased flexion of the spine. The pressures moved, but were not significantly reduced with stand-alone cages. 

New software helps create accurate 3D computer models that include shape and bone mineral density mapping from CT scanned bones. CT scans of cervical spines are being collected for the first application.

Cadaver arms instrumented with 2 mm plates on the proximal radius showed no impingement with prono-supination, but 2.4 mm and 3.5 mm plate did impinge. There was also no statistical difference in bending stiffness in 3 planes or failure torque between the 3.5 mm plate and 2 mm 90-90 plating construct.

Three different configurations of plate and screw fixation for mid shaft clavicle fractures and also a mid- shaft gap were compare in bending and torsion loading. 

The effect of screw angle on oblique fracture stability was studied in Sawbones cortical bone tubes simulating a mid shaft fracture of the radius.

Three different tibia plate and screw configurations were compared in a small mid-shaft gap model. Laboratory tests were used to validate a computer model of the motions allowed at the gap to assess the tissue strain patterns consistent with optimum healing. 

A locked intramedullary nail fixation of the small gap mid-shaft tibia model was compared to plate and screw model above. A computer model of this configuration is being completed to compare the tissue strains for this configuration to that of the plate a screw model.

The nail from the above study was then coated with bone cement, simulating the manner in which infected tibial fractures are treated with antibiotic impregnated cement over the nail. This also will be included in the computer model of the tibia and fixations of the mid-shaft gap.

A device is being developed to measure the tension in a cable system that is used for transporting a bone fragment along a diaphyseal defect in the tibia. The cable transport system is working well in treating patients with large bone defects, but it is not known how fast the bone can be transported minimize the treatment time for these patients. The measuring system will monitor the tension as the patient controls the bone transport, so the maximum safe transport speed can be defined.

DEXA scans define BMD distribution in the human femur. The goal is to find ways to predict fragility fracture risk in osteoporotic patients based on correlation of bone density measures, mechanical property measures of the bones and computer models that can be patient specific.

An unstable distal femoral fracture was simulated in cadaver femurs after fixation with a Depuy Synthes VA-LCP curved condylar plate. Cyclic loading caused fatigue failure in the plate through open screw holes in the plate, similar to clinical experiences. Filling open holes did not improve the fatigue life of the constructs.

Cadaver femurs were compared to a Sawbones mechanical femoral model for the decrease in strength of cortical bone due to stress concentration from screw holes. While the Sawbones model was substantially stronger than our cadaver bones, the percent reduction in strength due to screw holes was very similar to cadaver bones.

Gender differences of motion created by simulated single leg stance, double leg stance, flextion and extension loading and axial rotation loading on the lumbar spine and pelvix.

We tested two suture techniques for the supraspinatus repair. The standard method is fraught with complications from the repair site damaging the muscle that glides over the repair. Dr. Baraga’s technique protects the muscle after the repair, and our mechanical tests showed it provided the same strength of repair to the standard.

The most popular method to test rotator cuff repairs is fraught with inaccuracies that can invalidate conclusions from some of the published studies. So we used the “standard” measuring tools and measured what was really happening with markers that we could track on fluoroscopic images during testing  of the repairs in cadavers.

One popular way to prepare an autograft to repair a torn ACL is to take a small tendon, and create a graft using four strands of the tendon attached together. But the standard manner of attaching them does not allow the four strands to be evenly loaded. So we tested a new technique to show that the attachments can allow for more even loading and thus a stronger graft.

We tested specimens of decalcified bone matrix from the UM tissue bank to define their mechanical properties, and in particular their viscoelastic properties. In previous studies we showed that ACLs have very different visco-elastic properties from the tendon grafts that are being used to repair them. The tendon grafts can stretch with time and lose their tension on the knee. Decalcified bone grafts are much closer to normal ACLs in their ability to resist such stretching over time. 

A study is being developed to evaluate the effect of qadracepts graft location on patella-femoral mechanics.

ACL graft preparations from Achilles tendons compared a mid-line split of a tissue bank tendon, to a selected mid-substance dissection of the thickest portion of the tendon. The mid substance portion was stronger than the mid-line split grafts in matched paired samples. 

A major grant provides for a series of studies aimed at developing a novel computational model of the meniscal tissue which will guide scientists to engineer constructs mimicking electro-mechanical and transport properties of the native tissue.

The triquetral rotational index (TRI) is evaluated as an image analysis technique for diagnosis of ulnocarpal wrist instability in 105 healthy volunteers and 120 fresh human cadaver arms.*

Pathology of the TFCC is common cause of ulnar wrist pain but may also present with instability. In a cadaver study, we have demonstrated decreased stiffness in the UC joint after a peripheral-sided TFCC tear.*

Ulnar triquetral resistance to movement was measured in cadaver forearms before and after simulated TFCC tear, after stabilization with a functional brace designed for fracture treatment, and then again after surgical repair of the TFCC. The brace provided significant support to the injured TFCC.*

With supination tests of 30 cadaver arms, the examiner rated 4 wrists as stable and 26 as unstable. In biomechanical testing, the wrists judged to be stable had an average ulno-pisiform motion of 2.0 mm ±0.54 and wrists judged unstable had an average motion of 3.58 mm ±1.26. There was a statistically significant difference between the motion of the “stable” vs. “unstable” wrists.*

Carpal tunnel release has some potential harmful side effects from nerve damages during the arthroscopic procedure. We made ultra sonic images of carpal tunnel procedures in cadaver arms to track the stretches and mechanical interactions of the procedure with target nerves. One promising method to avoid these injuries is to introduce a balloon catheter to gently move nerves out of the way of the cutting portion of the procedure.

Radial-ulnar joint laxity and ulno-carpal laxity before and after simulated TFCC injury and after repair by extensor retinaculum capsulorrhapy v Adams procedure was studied in cadaver arms.

Prophylactic nailing of proximal femoral defect from tumors is under development. A pilot study is being carried out to evaluate a new method to measure surface strains on the bone both before and after creating the defect and then after various methods of prophylactic nailing to protect the defect until it can heal.

Patellofemoral joint mechanics was studied under muscle loading on cadaver knees at various knee positions with the natural patella in place and a total knee prosthesis in place. The goal was to see how the femoral component (designed to articulate with a patella prosthesis) engaged with natural patella by mapping the contact pressures between the patella and the prosthesis. The surgeons are performing this procedure now without using the patella prosthesis.

When preparing the cuts for application the femoral component of total knee, the fixtures that control the cuts can often be slightly mal-aligned and cause a notch in the anterior portion of the femoral condyle that can weaken the bone an increase the potential for mechanical failure. The accuracy of cuts was analyzed in Sawbones models comparing the cuts made by experienced joint surgeons and surgeons in training. 

What factors with previous lumbar spine and/or sacroiliac surgery will affect the safe zone for placement of THR components. This project is under development to understand the interactions of the lumbar spine and the hip joint.

Wiring paraprosthetic split in the femur during total hip replacement has been fraught with many post operative complications. We looked at the tension in the cable used to hold the split femur and also the location of this cable after loading cadaver femurs with a simulated split and a total hip femoral component in place.

The use of locked plate fixation with distal locked screw v wire after periprosthetic fracture in the distal femur was studied in cadaver femurs. The wire fixation in the distal end of the plate was stronger in torsion.

Diagnosis of traumatic ankle arthrotomies using saline load tests v methylene blue and radiopaque tracer was studied in cadavers with simulated medial malleolar fractures.

Lateral column lengthening osteotomy with prefabricated wedges was carried out in cadaver models in order to measure the movement of the calcaneus segments related to each wedge size.

Several cadaver legs were carefully dissected to define the anatomic structures in and around the syndemosis. A superficial thickening connecting the calcaneous to the distal fibula was identified in several cadavers, and appeared to be an “extra ligament” in some legs. But this was inconsistent in several legs investigated. In one leg the ankle was casted in several positions of the ankle and subtalar joints so we could hold those positions during MRI of the foot and ankle in each position. The goal was to define which soft tissue structures were in tension and which were relaxed in each position.

The incidence of injury to the calcaneofibular ligament in following severe ankle sprains was simulated in cadaver limbs.