Mechanical Properties of Fresh, Frozen and Vitrified Articular Cartilage.

Osteochondral allograft transplantations are typically used to treat focal articular cartilage injuries where the damaged cartilage is replaced with fresh cadaveric donor grafts. Despite the notable success rate of this procedure, it is limited by fresh donor tissue availability which can only be stored for approximately 28 days after harvest. Vitrification, a form of cryopreservation, can extend the storage time of cartilage. Although it has shown to preserve chondrocyte viability, its effect on the mechanical properties of the tissue has not been thoroughly investigated. Therefore, in this study, the mechanical properties of fresh, frozen, and vitrified articular cartilage were evaluated through unconfined compression testing. Results showed that the peak modulus, equilibrium modulus, and relaxation time constants of the vitrified and control samples (tested one day after harvest) were similar and higher than the fresh (tested 21 days after harvest) and frozen samples. This demonstrated that vitrification does not adversely affect the mechanical properties of cartilage and can be used as an alternative to fresh allografts which are limited by storage time. The fresh samples also had inferior mechanical properties compared to the control samples suggesting that vitrified allografts could potentially improve clinical outcomes in addition to increasing donor tissue availability.

Vitrified Particulated Articular Cartilage for Joint Resurfacing: A Swine Model.

BACKGROUND: The use of particulated articular cartilage for repairing cartilage defects has been well established, but its use is currently limited by the availability and short shelf life of donor cartilage. Vitrification is an ice-free cryopreservation technology at ultralow temperatures for tissue banking. An optimized vitrification protocol has been developed for particulated articular cartilage; however, the equivalency of the long-term clinical efficacy of vitrified particulated articular cartilage compared with fresh articular cartilage has not yet been determined. HYPOTHESIS: The repair effect of vitrified particulated cartilage from pigs would be equivalent to or better than that of fresh particulated cartilage stored at 4°C for 21 days. STUDY DESIGN: Controlled laboratory study. METHODS: A total of 19 pigs were randomly divided into 3 experimental groups: fresh particulated cartilage group (n = 8), vitrified particulated cartilage group (n = 8), and negative control group (no particulated cartilage in the defect; n = 3). An additional pig was used as the initial cartilage donor for the first set of surgical procedures. Pigs were euthanized after 6 months to obtain femoral condyles, and the contralateral condyle was used as the positive (no defect) control. Samples were evaluated for gross morphology using the Outerbridge and Osteoarthritis Research Society International (OARSI) scoring systems, histology (safranin O, collagen type I/II, DAPI), and chondrocyte viability using live-dead membrane integrity staining. RESULTS: There were no infections after surgery, and all 19 pigs were followed for the duration of the study. The OARSI grades for the fresh and vitrified particulated cartilage groups were 2.44 ± 1.35 and 2.00 ± 0.80, respectively, while the negative control group was graded significantly higher at 4.83 ± 0.29. Analysis of histological and fluorescent staining demonstrated that the fresh and vitrified particulated cartilage groups had equivalent regeneration within cartilage defects, with similar cell viability and densities and expression of proteoglycans and collagen type I/II. CONCLUSION: The implantation of fresh or vitrified particulated cartilage resulted in the equivalent repair of focal cartilage defects when evaluated at 6 months after surgery. CLINICAL RELEVANCE: The vitrification of particulated cartilage is a viable option for long-term storage for cartilage tissue banking and could greatly increase the availability of donor tissue for transplantation.

Prediction of fracture initiation and propagation in pelvic bones.

The objective is developing an XFEM model that is capable of predicting different types of fracture in the pelvic bone under various loading conditions. Previously published mechanical and failure characteristics of cortical and cancellous tissues were implemented and assigned to an intact pelvic bone with specified cortical and cancellous tissues. Various loading conditions, including combined load directions, were applied to the acetabulum to model different types of fracture (e.g., anterior/posterior wall fracture and transverse fracture) in the pelvic bone. The predicated types of fracture and the maximum force at fracture were compared to those acquired from previously published experimental tests. Anterior/posterior wall fracture and transverse fracture were the most common types of fractures determined in the simulations. The XFEM simulations were able to predict similar fractures to those reported in the experimental tests. The maximum fracture force in the XFEM model was found to be 18.6 kN compared to 8.85 kN reported in the previous experimental tests. The results revealed that different types of fracture in the pelvic bones can be caused by the various loading conditions in unstable high-rate impact loads. Using proper mechanical and failure behaviors of cortical and cancellous tissues, XFEM modeling of pelvic bone is capable of predicting bone fracture. In future work, the XFEM models of cancellous and cortical tissues can be assigned to other bones in human body skeleton so that the failure mechanism in such bones can be investigated.

Polycarbonate-urethane coating can significantly improve talus implant contact characteristics.

Talus implants can be utilized in cases of talus avascular necrosis and has been regarded as a promising treatment method. However, existing implants are made of stiff materials that directly oppose natural cartilage. The risk of long-term cartilage wear and bone fracture from the interaction between the cartilage and stiff implant surfaces has been documented in post-hemiarthroplasty of the hip, knee and ankle joints. The aim is to explore the effects of adding a layer of compliant material (polycarbonate-urethane; PCU) over a stiff material (cobalt chromium) in talus implants. To do so, we obtained initial ankle geometry from four cadaveric subjects in neutral standing to create the finite element models. We simulated seven models for each subject: three different types of talus implants, each coated with and without PCU, and a biological model. In total, we constructed 28 finite element models. By comparing the contact characteristics of the implant models with their respective biological model counterparts, our results showed that PCU coated implants have comparable contact area and contact pressure to the biological models, whereas stiff material implants without the PCU coating all have relatively higher contact pressure and smaller contact areas. These results confirmed that adding a layer of compliant material coating reduces the contact pressure and increases the contact area which in turn reduces the risk of cartilage wear and bone fracture. The results also suggest that there can be clinical benefits of adding a layer of compliant material coating on existing stiff material implants, and can provide valuable information towards the design of more biofidelic implants in the future.

Quantitative analysis of regional specific pelvic symmetry.

Understanding bilateral pelvic symmetry can be useful for analyzing complex pelvis anatomy and simplifying difficult procedures for pelvic fractures. This paper aims to quantify the degree of regional pelvic symmetry using computer-based methods. CT scans of 30 intact pelvises were digitized into 3D models and regions were defined: the ilium, acetabulum, pubis, and ischium. The right hemipelvis was aligned with the left, and deviations between the two models were quantified using method 1 (global registration) and method 2 (local registration). Symmetry was evaluated using the root mean square (RMS) of the deviations and the percentage of points within preset thresholds of ± 2 mm and ± 1 mm. The results showed that > 86% of points are within the ± 2 mm deviation threshold and average RMS are < 1.33 mm. For all regions, method 2 showed lower deviations than method 1. The pubis and ischium regions showed a large difference in symmetry between the two methods indicating high local symmetry, but a degree of global asymmetry. Conversely, the acetabular and iliac regions showed similar levels of symmetry with the two methods. When evaluated locally, the pelvic regions can be considered highly symmetric; the acetabulum is highly symmetric globally as well. These findings can be used in future studies to assess the feasibility of patient-specific implants using the mirrored contralateral hemipelvis as a template for unilateral pelvic fracture fixation. The left image shows the "cut planes" used to define four pelvic regions: the ilium, acetabulum, pubis, and ischium. The right image shows a deviation color map (DCM) used to quantify bilateral pelvic symmetry. The scale and color illustrate the degree of deviation of the left hemipelvis with the right hemipelvis with the units in millimeters (mm).

Development and application of the average pelvic shape in virtual pelvic fracture reconstruction.

BACKGROUND: With unilateral pelvic fractures, the contralateral hemipelvis can be used as a template in virtual reconstruction; however, this cannot be applied for bilateral fractures. Therefore, statistical shape modelling was used to build average pelvic shapes that can serve as templates when reconstructing bilaterally fractured pelvises. METHODS: Four average shape models were created for male and female, left and right hemipelves from 20 male and 20 female subjects. They were used as templates to reconstruct eight unilaterally fractured pelvises. RESULTS: The average root-mean-square of deviations between the reconstructed and intact hemipelves was 1.46 ± 0.32 mm, which is less than the 2 mm threshold for causing hip joint complications. CONCLUSION: This indicates that the reconstructions are reliable and the average shape models can be used to reconstruct bilaterally fractured pelvises. The proposed technique can potentially provide quick and accurate treatment plans for pelvic fracture patients, which is necessary for recovery.

Prediction of failure in cancellous bone using extended finite element method.

The objective of our study is to develop extended finite element method models of cancellous bone specimens that are capable of accurately predicting the onset and propagation of cracks under mechanical loading. In order to do so, previously published three-point bending test results of a single trabecula were replicated using two different extended finite element method approaches, namely, elastic-plastic-fracture and elastic-fracture that considered different configurations of the elasto-plastic properties of bone from which the best approach to fit the experimental data was identified. The behavior of a single trabecula was then used in 2D extended finite element method models to quantify the strength of the trabecular tissue of the forearm along three perpendicular anatomical axes. The results revealed that the elastic-plastic-fracture model better represented the experimental data in the model of a single trabecula. Considering the 2D trabecular specimens, the elastic fracture model predicted higher strength than the elastic-plastic-fracture model and there was no difference in stiffness between the two models. In general, the specimens exhibited higher failure strain and more ductile behavior in compression than in tension. In addition, strength and stiffness were found to be higher in tension than compression on average. It can be concluded that with proper parameters, extended finite element method is capable of simulating the ductile behavior of cancellous bone. The models are able to quantify the tensile strength of trabecular tissue in the various anatomical directions reporting an increased strength in the longitudinal direction of forearm cancellous bone tissue. Extended finite element method of cancellous bone proves to be a valuable tool to predict the mechanical characteristics of cancellous bones as a function of the microstructure.

An Equivalent Constitutive Model of Cancellous Bone With Fracture Prediction.

To simulate the mechanical and fracture behaviors of cancellous bone in three anatomical directions and to develop an equivalent constitutive model. Microscale extended finite element method (XFEM) models of a cancellous specimen were developed with mechanical behaviors in three anatomical directions. An appropriate abaqus macroscale model replicated the behavior observed in the microscale models. The parameters were defined based on the intermediate bone material properties in the anatomical directions and assigned to an equivalent nonporous specimen of the same size. The equivalent model capability was analyzed by comparing the micro- and macromodels. The hysteresis graphs of the microscale model show that the modulus is the same in loading and unloading; similar to the metal plasticity models. The strength and failure strains in each anatomical direction are higher in compression than in tension. The microscale models exhibited an orthotropic behavior. Appropriate parameters of the cast iron plasticity model were chosen to generate macroscale models that are capable of replicating the observed microscale behavior of cancellous bone. Cancellous bone is an orthotropic material that can be simulated using a cast iron plasticity model. This model is capable of replicating the microscale behavior in finite element (FE) analysis simulations without the need for individual trabecula, leading to a reduction in computational resources without sacrificing model accuracy. Also, XFEM of cancellous bone compared to traditional finite element method proves to be a valuable tool to predict and model the fractures in the bone specimen.

Virtual reconstruction of unilateral pelvic fractures by using pelvic symmetry.

PURPOSE: Pelvic fractures are known to be one of the most difficult injuries to treat. The objective of this study is to introduce a novel technique for virtual unilateral pelvic fracture reconstruction. Since the pelvis exhibits remarkable left-right symmetry, the contralateral hemipelvis can be used as a template for rebuilding the fractured hemipelvis. METHODS: CT scan data of the pelvic region of eight subjects with acute unilateral pelvic fractures were involved in this study. Computer-aided design software was used to create 3D models of these pelvises. The contralateral hemipelvis of each subject was then reflected across the sagittal plane, and the fractured hemipelvis was rebuilt by aligning the bone fragments with their equivalent location on the reflected side. To evaluate the quality of this reduction process, a 3D deviation analysis was conducted to calculate the differences between the reflected intact hemipelvis and the reconstructed hemipelvis. RESULTS: Results showed that the average root mean square (RMS) of deviations and average percentage of points within a ± 2 mm predefined threshold was 1.32 ± 0.22 mm and 88.4 ± 3.78%, respectively. The deviation color maps obtained indicated that the largest differences were along the fracture lines and on the non-articular surfaces of the pelvises. CONCLUSION: These results allowed us to conclude the validity of this procedure, since the average RMS difference was below 2 mm and the average percentage of points within ± 2 mm was high. The proposed technique will allow surgeons to provide their patients with more accurate reconstruction procedures which can potentially improve surgical outcomes.

Investigation of pelvic symmetry using CAD software.

Severe pelvic fractures often prove difficult for surgeons as they require patient-specific surgical treatment plans and customized equipment. Developing virtual patient-specific 3D pelvis models would ease the surgical planning process and enable development of custom fixation plates. This paper aims to examine pelvic symmetry to conclude whether the contralateral side may be used as a reference model for the fractured side of the pelvis. Fourteen subjects with intact pelvises were involved in this study. CT scans of the pelvises were converted to 3D models and the right sides of the pelvises were reflected and aligned with the left sides. A deviation analysis was then performed for each set of models and results showed that the average root mean square (RMS) of values was 1.14 ± 0.26 mm and the average percentage of points below a deviation threshold of ± 2 mm was 91.9 ± 5.55%. The deviation color maps (DCMs) showed that the largest deviations were on the non-articular surfaces. The volume and surface area of each model were also examined and showed no significant differences between left and right sides. These results indicate that the pelvis displays bilateral symmetry and this concept can be used to develop fully intact patient-specific 3D pelvis models for fracture reconstruction using the unfractured contralateral side. Graphical Abstract.

Biomechanical evaluation of the Nice knot.

BACKGROUND: The Nice knot is a bulky double-stranded knot. Biomechanical data supporting its use as well as the number of half hitches required to ensure knot security is lacking. MATERIALS AND METHODS: Nice knots with, one, two, or three half-hitches were compared with the surgeon's and Tennessee slider knots with three half hitches. Each knot was tied 10 times around a fixed diameter using four different sutures: FiberWire (Arthrex, Naples, FL), Ultrabraid (Smith and Nephew, Andover, MA), Hi-Fi (ConMed Linvatec, Largo, FL) and Force Fiber (Teleflex Medical OEM, Gurnee, IL). Cyclic testing was performed for 10 min between 10N and 45N, resulting in approximately 1000 cycles. Displacement from an initial 10N load was recorded. Knots surviving cyclic testing were subjected to a load to failure test at a rate of 60 mm/min. Load at clinical failure: 3 mm slippage or opening of the suture loop was recorded. Bulk, mode of ultimate failure, opening of the loop past clinical failure, was also recorded. RESULTS: During cyclic testing, the Nice knots with one or more half-hitches performed the best, slipping significantly less than the surgeon's and Tennessee Slider (P < 0.002). After one half-hitch, the addition of half-hitches did not significantly improve Nice knot performance during cyclic testing (P > 0.06). The addition of half-hitches improved the strength of the Nice knot during the force to failure test, however after two half-hitches, increase of strength was not significant (P = 0.59). While FiberWire was the most bulky of the sutures tested, it also performed the best, slipping the least. CONCLUSION: The Nice knot, especially using FiberWire, is biomechanically superior to the surgeon's and Tennessee slider knots. Two half hitches are recommended to ensure adequate knot security.

A geometric approach to study the contact mechanisms in the patellofemoral joint of normal versus patellofemoral pain syndrome subjects.

The biomechanics of the patellofemoral (PF) joint is complex in nature, and the aetiology of such manifestations of PF instability as patellofemoral pain syndrome (PFPS) is still unclear. At this point, the particular factors affecting PFPS have not yet been determined. This study has two objectives: (1) The first is to develop an alternative geometric method using a three-dimensional (3D) registration technique and linear mapping to investigate the PF joint contact stress using an indirect measure: the depth of virtual penetration (PD) of the patellar cartilage surface into the femoral cartilage surface. (2) The second is to develop 3D PF joint models using the finite element analysis (FEA) to quantify in vivo cartilage contact stress and to compare the peak contact stress location obtained from the FE models with the location of the maximum PD. Magnetic resonance images of healthy and PFPS subjects at knee flexion angles of 15°, 30° and 45° during isometric loading have been used to develop the geometric models. The results obtained from both approaches demonstrated that the subjects with PFPS show higher PD and contact stresses than the normal subjects. Maximum stress and PD increase with flexion angle, and occur on the lateral side in healthy and on the medial side in PFPS subjects. It has been concluded that the alternative geometric method is reliable in addition to being computationally efficient compared with FEA, and has the potential to assess the mechanics of PFPS with an accuracy similar to the FEA.

Three-dimensional geometric analysis of the talus for designing talar prosthetics.

Proper understanding of the complex geometric shape of the talus bone is important for the design of generic talar body prosthetics and restoration of the proper ankle joint function after surgery. To date, all talus implants have been patient-specific with the limitation that complex computer modeling is required to produce a mirrored image from the unaffected opposite side followed by machining a patient-specific prosthesis. To develop an "off-the-shelf" non-custom talar prosthesis, it is important to perform a thorough investigation of the geometric shape of the talus bone. This article addresses the applicability of a scaling approach for investigating the geometric shape and similarity of talus bones. This study used computed tomography scan images of the ankle joints of 27 different subjects to perform the analysis. Results of the deviation analyses showed that the deviation in the articulating surfaces of the talus bones was not excessive in terms of talus size. These results suggest that a proposed range of five implant sizes is possible. Finally, it is concluded that the talus bones of the ankle joints are geometrically similar, and a proposed range of five implant sizes will fit a wide range of subjects. This information may help to develop generic talus implants that might be applicable to patients with a severe talus injury.

Improving greater trochanteric reattachment with a novel cable plate system.

Cable-grip systems are commonly used for greater trochanteric reattachment because they have provided the best fixation performance to date, even though they have a rather high complication rate. A novel reattachment system is proposed with the aim of improving fixation stability. It consists of a Y-shaped fixation plate combined with locking screws and superelastic cables to reduce cable loosening and limit greater trochanter movement. The novel system is compared with a commercially available reattachment system in terms of greater trochanter movement and cable tensions under different greater trochanteric abductor application angles. A factorial design of experiments was used including four independent variables: plate system, cable type, abductor application angle, and femur model. The test procedure included 50 cycles of simultaneous application of an abductor force on the greater trochanter and a hip force on the femoral head. The novel plate reduces the movements of a greater trochanter fragment within a single loading cycle up to 26%. Permanent degradation of the fixation (accumulated movement based on 50-cycle testing) is reduced up to 46%. The use of superelastic cables reduces tension loosening up to 24%. However this last improvement did not result in a significant reduction of the grater trochanter movement. The novel plate and cables present advantages over the commercially available greater trochanter reattachment system. The plate reduces movements generated by the hip abductor. The superelastic cables reduce cable loosening during cycling. Both of these positive effects could decrease the risks related to grater trochanter non-union.

The effects of femoral neck cut, cable tension, and muscles forces on the greater trochanter fixation.

Greater trochanter (GT) stabilization techniques following a fracture or an osteotomy are still showing high levels of postoperative complications. Understanding the effect of femoral neck cut placement, cable tension and muscles forces on GT fragment displacements could help surgeons optimize their techniques. A 3D finite element model has been developed to evaluate, through a statistical experimental design, the impact of the above variables on the GT fragment gap and sliding displacements. Muscles forces were simulating typical daily activities. Stresses were also investigated. The femoral neck cut placement had the most significant effect on the fragment displacement. Lowering it by 5 mm increased the gap and sliding fragment displacements by 288 and 128 %, respectively. Excessive cable tightening provided no significant reduction in fragment displacement. Muscle activities increased the gap and the sliding displacements for all muscle configurations. The maximum total displacement of 0.41 mm was present with a 10 mm femoral neck cut, a cable tension of 178 N, and stair climbing. Caution must be used not to over tighten the cables as the potential damage caused by the increased stress is more significant than any reduction in fragment displacement. Furthermore, preservation of the contact area is important for GT stabilization.

Biomechanical analysis of chlorhexidine power irrigation to disinfect contaminated anterior cruciate ligament grafts.

BACKGROUND: Accidental graft contamination is a likely complication to occur in an orthopaedic sports medicine surgeon's career. There are no clinical outcome studies to direct management, and a recent survey showed that preferred management varied. Three liters of 2% chlorhexidine power irrigation has been shown to be an efficient and effective disinfection protocol; however, the biomechanical sequelae of this disinfection protocol to the graft are unknown. PURPOSE: The purpose of this study was to determine if 3 L of 2% chlorhexidine power irrigation used to disinfect contaminated anterior cruciate ligament (ACL) grafts significantly weakens the graft at time zero. STUDY DESIGN: Controlled laboratory study. METHODS: Eight fresh bovine superficial digital flexor tendons underwent disinfection protocol with 3 L of 2% chlorhexidine power irrigation. Contralateral tendons served as the control. Tendons were preconditioned and loaded to failure. RESULTS: The average ultimate failure load for the control tendons and disinfected tendons was 4081 ± 654.4 N and 4146 ± 723.2 N, respectively. The average ultimate failure stress for the control tendons and disinfected tendons was 97.10 ± 12.03 MPa and 95.18 ± 11.79 MPa, respectively. The average stiffness for the control tendons and disinfected tendons was 546.20 ± 28.16 N/mm and 539.2 ± 25.93 N/mm, respectively. The P values for ultimate failure load, ultimate failure stress, and stiffness showed no statistically significant difference between the chlorhexidine and control tendons. CONCLUSION: Disinfecting tendons with 3 L of 2% chlorhexidine power irrigation does not adversely weaken the tendon's tensile mechanical properties. CLINICAL RELEVANCE: Three liters of 2% chlorhexidine power irrigation can effectively disinfect a contaminated ACL graft without weakening the graft.

Effect of force tightening on cable tension and displacement in greater trochanter reattachment.

The purpose of this study was to evaluate cable tension during installation, and during loading similar to walking in a cable grip type greater trochanter (GT), reattachment system. A 4th generation Sawbones composite femur with osteotomised GT was reattached with four Cable-Ready® systems (Zimmer, Warsaw, IN). Cables were tightened at 3 different target installation forces (178, 356 and 534 N) and retightened once as recommended by the manufacturer. Cables tension was continuously monitored using in-situ load cells. To simulate walking, a custom frame was used to apply quasi static load on the head of a femoral stem implant (2340 N) and abductor pull (667 N) on the GT. GT displacement (gap and sliding) relative to the femur was measured using a 3D camera system. During installation, a drop in cable tension was observed when tightening subsequent cables: an average 40+12.2% and 11 ± 5.9% tension loss was measured in the first and second cable. Therefore, retightening the cables, as recommended by the manufacturer, is important. During simulated walking, the second cable additionally lost up to 12.2+3.6% of tension. No difference was observed between the GT-femur gaps measured with cables tightened at different installation forces (p=0.32). The GT sliding however was significantly greater (0.9 ± 0.3 mm) when target installation force was set to only 178 N compared to 356 N (0.2 ± 0.1 mm); p<0.001. There were no significant changes when initial tightening force was increased to 534 N (0.3 ± 0.1 mm); p=0.11. In conclusion, the cable tightening force should be as close as possible to that recommended by the manufacturer, because reducing it compromises the stability of the GT fragment, whereas increasing it does not improve this stability, but could lead to cable breakage.

Dynamic positioning of scoliotic patients during spine instrumentation surgery.

STUDY DESIGN: Evaluation of a novel Dynamic Positioning Frame (DPF) for scoliosis surgery to improve presurgical correction. OBJECTIVE: To assess a DPF for scoliosis surgery and demonstrate how its design offers additional presurgical correction of the 3-dimensional deformity. SUMMARY OF BACKGROUND DATA: Patient positioning is an important step in scoliosis surgery. Although current positioning frames focus on supporting the patient and keeping the abdomen pendulous to reduce blood loss, not much has been carried out to explore the aspect of dynamic positioning during surgery. When lying prone, there is some spontaneous correction of the scoliotic deformity owing to gravity and anesthesia. METHODS: Trunk 3-dimensional geometry and pressure at the patient-cushion interface were measured for 12 unanesthetized patients in various positions-standing (PI), lying prone on the DPF (PII), lying prone on the DPF with applied corrective forces (PIII), and lying prone on the Relton-Hall (R-H) frame for comparison (PIV). RESULTS: Spine height increased significantly in prone as compared with that in standing position. When lying on the DPF with corrective forces, there was an improvement in the patients' transverse plane deformity and rib hump with greater retention of kyphosis. There was also an improvement in the rib hump as compared with that in the R-H frame. Higher pressures were recorded on the DPF as compared with the R-H frame, but were reduced to similar values when larger cushions were used. CONCLUSIONS: The DPF provides a novel way of modifying the patient's position preoperatively and intraoperatively. Dynamic patient positioning, coupled with applied corrective forces, allows for increased reduction of the scoliotic deformity as compared with the R-H frame. Further investigation is required to optimize cushion placement and thus, insure safe patient-cushion interface pressures.

Computer simulation for the optimization of patient positioning in spinal deformity instrumentation surgery.

Studies have shown that scoliosis curves correct when patients are positioned on the operating table prior to instrumentation. However, biomechanical aspects of positioning have not been widely studied. The objective of this study was to simulate patient positioning during instrumentation surgery and test various adjustment parameters of the trunk and recommend optimal patient positioning prior to, and during spine surgery based on the results of finite element simulations. A scoliotic patient was simulated using a finite element model and six different positioning parameters were modified while ten geometric measures were recorded. Statistical analysis determined which model parameter had a significant effect on the geometric measures. Geometric measures were individually and simultaneously optimized, while corresponding model parameters were documented. Every model parameter had a significant effect on at least five of the geometric measures. When optimizing a single measure, others would often deteriorate. Simultaneous optimization resulted in improved overall correction of the patient's geometry by 75% however ideal correction was not possible for every measure. Finite element simulations of various positioning parameters enabled the optimization of ten geometric measures. Positioning is an important surgical step that should be exploited to achieve maximum correction.

Biomechanical simulations of scoliotic spine correction due to prone position and anaesthesia prior to surgical instrumentation.

BACKGROUND: The positioning of patients during scoliosis surgery has been shown to affect the scoliosis curve, yet positioning has not been exploited to help improve surgical outcome from a biomechanics point of view. Biomechanical models have been used to study other aspects of scoliosis. The goal of this study is to simulate the specific influence of the prone operative position and anaesthesia using a finite element model with patient personalized material properties. METHODS: A finite element model of the spine, ribcage and pelvis was created from the 3D standing geometry of two patients. To this model various positions were simulated. Initially the left and right supine pre-operative bending were simulated. Using a Box-Benkin experimental design the material properties of the intervertebral disks were personalized so that the bending simulations best matched the bending X-rays. The prone position was then simulated by applying the appropriate boundary conditions and gravity loads and the 3D geometry was compared to the X-rays taken intra-operatively. Finally an anaesthesia factor was added to the model to relax all the soft tissues. FINDINGS: The behaviour of the model improved for all three positions once the material properties were personalized. By incorporating an anaesthesia factor the results of the prone intra-operative simulation better matched the prone intra-operative X-ray. However, the anaesthesia factor was different for both patients. For the prone position simulation with anaesthesia patient 1 corrected from 62 degrees to 47 degrees and 43 degrees to 31 degrees. Patient 2 corrected from 70 degrees to 55 degrees and 40 degrees to 32 degrees for the thoracic and lumbar curves respectively. INTERPRETATION: Positioning of the patient, as well as anaesthesia, provide significant correction of the spinal deformity even before surgical instrumentation is fixed to the vertebra. The biomechanical effect of positioning should be taken into consideration by surgeons and possibly modify the support cushions accordingly to maximise 3D curve correction. The positioning is an important step that should not be overlooked by when simulating surgical correction and biomechanical models could be used to help determine optimal cushion placement.