Aims

Research on hip biomechanics has analyzed femoroacetabular contact pressures and forces in distinct hip conditions, with different procedures, and used diverse loading and testing conditions. The aim of this scoping review was to identify and summarize the available evidence in the literature for hip contact pressures and force in cadaver and in vivo studies, and how joint loading, labral status, and femoral and acetabular morphology can affect these biomechanical parameters.

Purpose. To validate a small, easy to use and cost-effective augmented marker-based hybrid navigation system for peri-acetabular osteotomy [PAO] surgery. Methods. A cadaver study including 3 pelvises (6 hip joints) undergoing navigated PAO was performed. Inclination and anteversion of two navigation systems for PAO were compared during acetabular reorientation. The hybrid system consists of a tracking unit which is placed on the patient's pelvis and an augmented marker which is attached to the patient's acetabular fragment. The tracking unit sends a video stream of the augmented marker to the host computer. Simultaneously, the augmented marker sends orientation output from an integrated inertial measurement unit (IMU) to the host computer. The host computer then computes the pose of the augmented marker and uses it (if visible) to compute acetabular orientation. If the marker is not visible, the output from the IMU is used to update the orientation. The second system served as ground truth and is a previously developed and validated optical tracking-based navigation system. Results. Mean absolute difference for inclination and anteversion (N = 360) was 1.34 degrees and 1.21 degrees, respectively. The measurements from our system show a very strong correlation to the ground-truth optical tracking-based navigation system for both inclination and anteversion (0.9809 / 0.9711). Conclusion. In this work, we successfully demonstrated the feasibility of our system to measure inclination and anteversion during acetabular reorientation

Aims. Accurate placement of acetabular and femoral stem components in total hip arthroplasty (THA) is an important factor in the success of the procedure. A variety of free hand or navigated techniques is reported. Survivorship and complications have been shown to be directly related to implant position during THA. The aim of this cadaver study was to assess the accuracy of the placement of the components in THA using patient specific instruments (PSI) in combination with a 3D planning software and the direct anterior approach. Method. Patient specific instruments (PSI) were developed to guide the surgeon during THA that were 3D printed with their bone models following a 3D software planning protocol (LPH software V2.5.1, Onefit-Medical, Eos Imaging Company, Besancon, France). Acetabular guides: cup, offset and straight reamer handle and impactor, femoral- and chisel guides were used in each THA (Fig. 1). To define anatomic bone landmarks and to generate a 3D model of each hip joint CT scans were performed preoperatively. The planning of component position was done by one surgeon (AZ) preop. Surgery was performed by two experienced surgeons (AZ, SD) on cadaver specimen with 4 hips in two separate series. A total of 8 hip replacements were evaluated pre- and postoperatively using CT-scans of each hip joint to compare planned to achieved results. Mechanical simulations of the guides were carried out to verify that there were no conflicts between the different instruments. To meet the ISO standard 16061: 2015 the compatibility of the instruments with the guides has been checked. Parameters were evaluated in 3D pelvic and femoral planes: center cup position, inclination angle, anteversion angle, cutting height and plan orientation, anteversion angle, flexion/extension angle, varus/valgus angle, anatomical and functional leg length, offset. Acceptance criteria: postop. parameters evaluated must not have a deviation of more than 5 degrees, 2,5 mm according to preop. planning. For every THA the test protocol has been completely realized. Results. The difference between the preop. and postop. measures in the first series of 4 hips revealed 2 outliers because of fractures of the acetabulum in 2 cases, related to bad cadaver quality. In the second series we found satisfactory results comparing the planned preop and postop component position (Fig. 2). For example difference of leg length showed a mean absolute of 1,58 mm, standard deviation 1,21 mm (min 0,62; max 3,34 mm). Offset revealed a mean absolute of 1,62 mm, standard deviation 0,57 mm (min 1,06; max 2,14 mm) concerning the difference between preop. planning and result postop. Conclusion. Accurate and safe placement of total hip components in THA, both acetabular cup and stem, performing the direct anterior approach can be achieved using a 3D preoperative planning along with patient specific instruments. The results of the cadaver study tests are promising and that is to be proven in the clinical setting and by application in the future

Because ankle inversion trauma can result in persistent isolated subtalar joint instability and can contribute to chronic lateral ankle instability, optimization of subtalar joint ligament injury diagnosis and treatment is essential. 12 fresh-frozen cadaver lower extremities were used. The cradle was a component of a gimbal system that allowed unrestricted inversion/eversion and anterior-posterior and medial-lateral translation of the subtalar joint. The bearing system to which the tibia/fibula were attached allowed unconstrained internal/external rotation and superior-inferior translation. 4N-m inversion/ eversion and internal/external rotational moments and translational forces of 67N were applied. All measurements were performed sequentially in neutral, 10° dorsiflexion and 20° plantarflexion, and were repeated as the cervical, calcaneofibular, and interosseous ligaments were consecutively sectioned in all possible different orders. In neutral position, inversion increased after sectioning of the cervical (3.7°), interosseous (0.8°), and calcaneofibular (1.9°) ligaments individually. Combined sectioning of all three ligaments showed an increase in inversion of 8.3°, 8.5° and 1.4° in the neutral, plantarflexed, and dorsiflexed positions, respectively, compared to the intact ankle. External rotation also increased in neutral position after sectioning the cervical ligament (2.0°). Combined sectioning of all ligaments showed an increase in external rotation of 3.6° and 5.4° for neutral and dorsiflexion, respectively. This is the first comprehensive biomechanical cadaver study of the contributions of the cervical, calcaneofibular, and interosseous ligaments to stabilization of the subtalar joint. The surgeon may refer to the findings in both diagnosing and planning treatment of problematic subtalar joint instability

Objectives. To assess the accuracy of patient-specific instruments (PSIs) versus standard manual technique and the precision of computer-assisted planning and PSI-guided osteotomies in pelvic tumour resection. Methods. CT scans were obtained from five female cadaveric pelvises. Five osteotomies were designed using Mimics software: sacroiliac, biplanar supra-acetabular, two parallel iliopubic and ischial. For cases of the left hemipelvis, PSIs were designed to guide standard oscillating saw osteotomies and later manufactured using 3D printing. Osteotomies were performed using the standard manual technique in cases of the right hemipelvis. Post-resection CT scans were quantitatively analysed. Student’s t-test and Mann–Whitney U test were used. Results. Compared with the manual technique, PSI-guided osteotomies improved accuracy by a mean 9.6 mm (p < 0.008) in the sacroiliac osteotomies, 6.2 mm (p < 0.008) and 5.8 mm (p < 0.032) in the biplanar supra-acetabular, 3 mm (p < 0.016) in the ischial and 2.2 mm (p < 0.032) and 2.6 mm (p < 0.008) in the parallel iliopubic osteotomies, with a mean linear deviation of 4.9 mm (p < 0.001) for all osteotomies. Of the manual osteotomies, 53% (n = 16) had a linear deviation > 5 mm and 27% (n = 8) were > 10 mm. In the PSI cases, deviations were 10% (n = 3) and 0 % (n = 0), respectively. For angular deviation from pre-operative plans, we observed a mean improvement of 7.06° (p < 0.001) in pitch and 2.94° (p < 0.001) in roll, comparing PSI and the standard manual technique. Conclusion. In an experimental study, computer-assisted planning and PSIs improved accuracy in pelvic tumour resections, bringing osteotomy results closer to the parameters set in pre-operative planning, as compared with standard manual techniques. Cite this article: A. Sallent, M. Vicente, M. M. Reverté, A. Lopez, A. Rodríguez-Baeza, M. Pérez-Domínguez, R. Velez. How 3D patient-specific instruments improve accuracy of pelvic bone tumour resection in a cadaveric study. Bone Joint Res 2017;6:577–583. DOI: 10.1302/2046-3758.610.BJR-2017-0094.R1

Introduction. Knee instability, stiffness, and soft-tissue imbalance are causes of aseptic revision and patient dissatisfaction following total knee arthroplasty (TKA). Surgical techniques that ensure optimal ligament balance throughout the range of motion may help reduce TKA revision for instability and improve outcomes. We evaluated a novel tibial-cut first gap balancing technique where a computer-controlled tensioner is used to dynamically apply a varying degree of distraction force in real-time as the knee is taken through a range of motion. Femoral bone cuts can then be planned while visualizing the predicted knee implant laxity throughout the arc of flexion. Surgical Technique Description. After registering the mechanical axes and morphology of the tibia and femur using computer navigation, the tibial resection was performed and a robotic tensioning tool was inserted into the knee prior to cutting the femur. The tool was programmed to apply equal loads in the medial and lateral compartments of the knee, but to dynamically vary the distraction force in each compartment as the knee is flexed with a higher force being applied in extension and a progressively lower force applied though mid-flexion up to 90° of flexion. The tension and predictive femoral gaps between the tibial cut and the femoral component in real-time was determined based on the planned 3D position and size of the femoral implant and the acquired pre-resection gaps (figure 1). Femoral resections were then performed using a robotic cutting guide and the trial components were inserted. Methods. The technique was evaluated by three experienced knee arthroplasty surgeons on 4 cadaver knees (3 torso-to-toe specimens, Pre-operative deformity range: 4° varus − 6° valgus; Extension lag: 0° – 13°; BMI 23.4 – 32.6; Age 68 – 85yr). An applied targeted load of 80N in extension and 50N in flexion was used in each of the four knees. These force values were determined in a prior cadaver study aimed at determining what magnitude of applied load corresponded to an optimally rated knee tension and stability. The femoral component was planned in each of the four knees to have symmetric gaps at 0° and 90° of flexion. The overall balance of the knee was assessed clinically by each surgeon using a varus/valgus stress test with the trial components inserted. No soft-tissue releases were performed other than a standard medial release during initial exposure of the knee. The following scale was used to rate the final knee stability achieved: 1 – too loose; 2 – slightly loose; 3 – ideal; 4 slightly tight; 5 – too tight. Results. ‘Ideal' balance was achieved in three out of the four knees tested (table 1). In two of the four knees the final inserted thickness selected was 1mm thicker than the planned insert thickness. Conclusions. Our preliminary cadaver results suggest that it is possible to achieve a balanced knee by incorporating dynamic ligament tensioning and gap data throughout flexion into the femoral planning process using a robotic tensioning tool. For figures/tables, please contact authors directly.

Aims

One of the main causes of tibial revision surgery for total knee arthroplasty is aseptic loosening. Therefore, stable fixation between the tibial component and the cement, and between the tibial component and the bone, is essential. A factor that could influence the implant stability is the implant design, with its different variations. In an existing implant system, the tibial component was modified by adding cement pockets. The aim of this experimental in vitro study was to investigate whether additional cement pockets on the underside of the tibial component could improve implant stability. The relative motion between implant and bone, the maximum pull-out force, the tibial cement mantle, and a possible path from the bone marrow to the metal-cement interface were determined.

Introduction and Objective

Scapholunate instability is the most common cause of carpal instability. When this instability is left untreated, the mechanical relationship between the carpal bones is permanently disrupted, resulting in progressive degenerative changes in the radiocarpal and midcarpal joints. Different tenodesis methods are used in the treatment of acute or early chronic reducible scapholunate instability, where arthritis has not developed yet and the scapholunate ligament cannot be repaired. Although it has been reported that pain is reduced in the early follow up in clinical studies with these methods, radiological results differ between studies. The deterioration of these radiological parameters is associated with wrist osteoarthritis as previously stated. Therefore, more studies are needed to determine the tenodesis method that will improve the wrist biomechanics better and will last longer. In our study, two new tenodesis methods, spiral antipronation tenodesis, and anatomic front and back reconstruction (ANAFAB) were radiologically compared with triple ligament tenodesis (TLT), in the cadaver wrists.

Introduction: Femoral neck fractures are common and percutaneous insertion of three cannulated screws is an accepted method of surgical treatment. The accuracy of surgical performance is highly correlated with the cut-out percentages of the screws. The conventional technique relies heavily on fiuoroscopy and could lead to inappropriate implant placement. Further, multiple guidewire passes might prolong the operation time and weaken the cancellous bone. A computer-assisted planning and navigation system based on 2D-fiuoroscopy has been developed for guidewire insertion in order to perform insertion of a guidewire to perform screw insertion. The image acquisition process was supported by a radiation-saving procedure called “Zero-dose C-arm navigation”. The purpose of this experimental study was to compare this technique with conventional C-arm fiuoroscopy with respect to the number of fiuoroscopic images, the number of drilling attempts and operation time. We used two operative settings, with sawbones and with cadavers. For the sawbone study, we also compared the femoral neck and head perforation and the neck-width coverage (the relative area of the femoral neck held by screws). Methods: Three cannulated hip screws were inserted into 12 femoral sawbones simulating femoral neck fractures and into 6 cadaveric femurs guided by the computer-based navigation. We compared them to the conventional fiuoroscopic technique also using 12 femoral sawbones and 6 cadaveric femurs. Results: The computer-assisted technique significantly reduced the amount of intraoperative fiuoroscopy (sawbone study: P< 0.001; cadaver study: P< 0.001) and the number of guidewire passes (sawbone study: P< 0.05; cadaver study: P< 0.05) in the sawbone and the cadaver setting. Operation time was significantly longer (sawbone study: P< 0.001; cadaver study: P< 0.05) in the navigation assisted group also in both settings. In the sawbone study, there was no significant difference in the femoral neck and head perforation, whereas the relative neck area held by the screws was significantly (P< 0.05) larger than that in the conventional group. Discussion: The addition of computer-assisted planning and surgical guidance supported by “Zero-dose C-arm navigation” may be useful for the fixation of femoral neck fractures by cannulated screws as it reduces the amount of intraoperative fiuoroscopy, requires fewer drill tracks and achieves a better neck coverage. Further studies with the goal of reducing the operation time by improving the learning curve are indispensable before integrating this navigation system into the clinical workfiow

Direct anterior approach (DAA) is an inter-muscular approach that needs no muscle detached. In THA through DAA approach, exposure of the acetabulum is facilitated, while the key points of this approach are femoral lift-up and hip extension to get sufficient access to the femoral canal. To investigate the strategy for femoral lift-up, we released the capsule step by step and measured the distance of femoral lift-up at each step in cadavers and clinical cases. The effects of hip extension on femoral lift-up were also evaluated. Three fresh frozen cadavers were used. In supine position, the hip joint was exposed through DAA by two experienced surgeons. After anterior capsulotomy and femoral head resection, posterior capsule release was performed followed by superior capsule release in one side, and superior release was followed by posterior release in the other side. Finally, internal obturator muscle was released in both side. At each step, the distance of femoral lift-up was measured under the traction force of 70N. The effects of hip extension were investigated in 0, 15 and 25 degrees hyper-extension. Thirty-six THA were performed through DAA. Posterior capsule release was performed followed by superior capsule release in 13 hips, and superior release was followed by posterior release in 23 hips. At each step, the distance of femoral lift-up was measured under the traction force of 70N at each step same as the cadaver study. In cadaver study, anterior capsulotomy and posterior capsule release affected little the femoral lift-up. The distance increased after superior capsular release. The distance decreased as hip hyperextension unless the superior capsule was released. The effect of internal obturator muscle release was not observed. In clinical studies, the same tendency was observed in clinical cases. Superior capsule release was the most effective for the femoral lift-up. The results of this study indicate that superior capsule release is the first step for the femoral liftup. The second step is hip extension to get access to the femoral canal. By performing these procedures step by step, rasping and stem insertion can be achieved with minimal soft tissue release

Abstract

Optimal acetabular component position in Total Hip Arthroplasty is vital for avoiding complications such as dislocation and impingement, Transverse acetabular ligament (TAL) have been shown to be a reliable landmark to guide optimum acetabular cup position. Reports of iliopsoas impingement caused by acetabular components exist. The Psoas fossa (PF) is not a well-regarded landmark for Component positioning. Our aim was to assess the relationship of the TAL and PF in relation to Acetabular Component positioning.

A total of 22 cadavers were implanted on 4 occasions with the an uncemented acetabular component. Measurements were taken between the inner edge of TAL and the base of the acetabular component and the distance between the lower end of the PF and the most medial end of TAL.

The distance between the edge of the acetabular component and TAL was a mean of 1.6cm (range 1.4–18cm). The distance between the medial end of TAL and the lowest part of PF was a mean of 1.cm (range 1,3–1.8cm) It was evident that the edge of PF was not aligned with TAL.

Optimal acetabular component position is vital to the longevity and outcome following THA. TAL provides a landmark to guide acetabular component position. However we feel the PF is a better landmark to allow appropriate positioning of the acetabular component inside edge of the acetabulum inside the bone without exposure of the component rim and thus preventing iliopsoas impingement at the psoas notch and resultant groin pain.

Introduction. Posterolateral tibial plateau fractures account for 7 % of all proximal tibial fractures. Their fixation often requires posterolateral buttress plating. Approaches for the posterolateral corner are not extensile beyond the perforation of the anterior tibial artery through the interosseous membrane. This study aims to provide accurate data about the inferior limit of dissection by providing measurements of the anterior tibial artery from the lateral joint line as it pierces the interosseous membrane. Materials and Methods. Forty unpaired adult lower limbs cadavers were used. The posterolateral approach to the proximal tibia was performed as described by Frosch et al. Perpendicular measurements were made from the posterior limit of the articular surface of the lateral tibial plateau and fibula head to the perforation of the anterior tibial artery through the interosseous membrane. Results. The anterior tibial artery coursed through the interosseous membrane at 46.3 +/− 9.0 mm (range 27–62 mm) distal to the lateral tibial plateau and 35.7 +/− 9.0 mm (range 17–50 mm) distal to the fibula head. There was no significant difference between right or left sided knees. Discussion. This cadaveric study demonstrates the safe zone (min 27 mm, mean 45mm) up to which distal exposure can be performed for fracture manipulation and safe application of a buttress plate for displaced posterorlateral tibial plateau fractures. Evidence demonstrates quality of reduction correlates with clinical outcome and the surgeon can expect to be able to use a small fragment buttress plate of up to 45mm as this is the mean

To develop a useful surgical navigation system, accurate determination of bone coordinates and thorough understanding of the knee kinematics are important. In this study, we have verified our algorithm for determination of bone coordinates in a cadaver study using skeletal markers, and at the same time, we also attempted to obtain a better understanding of the knee kinematics. The research was performed at the Medical Simulation Center of Tzu Chi University. Optical measurement system (Polaris® Vicra®, Northern Digital Inc.) was used, and reflective skeletal markers were placed over the iliac crest, femur shaft, and tibia shaft of the same limb. Two methods were used to determine the hip center; one is by circumduction of the femur, assuming it pivoted at the hip center. The other method was to partially expose the head of femur through anterior hip arthrotomy, and to calculate the centre of head from the surface coordinates obtained with a probe. The coordinate system of femur was established by direct probing the bony landmarks of distal femur through arthrotomy of knee joint, including the medial and lateral epicondyle, and the Whiteside line. The tibial axis was determined by the centre of tibia plateau localised via direct probing, and the centre of ankle joint calculated by the midpoint between bilateral malleoli. Repeated passive flexion and extension of knee joint was performed, and the mechanical axis as well as the rotation axis were calculated during knee motion. A very small amount of motion was detected from the iliac crest, and all the data were adjusted at first. There was a discrepancy of about 16.7mm between the two methods in finding the hip centre, and the position found by the first method was located more proximally. When comparing the epicondylar axis to the rotation axis of the tibia around knee joint, there was a difference of 2.46 degrees. The total range of motion for the knee joint measured in this study was 0∼144 degrees. The mechanical axis was found changing in an exponential pattern from 0 degrees to undetermined at 90 degrees of flexion, and then returned to zero again. Taking the value of 5 degrees as an acceptable range of error, the calculated mechanical axis exceeded this value when knee flexion angle was between 60∼120 degrees. The discrepancy between the hip centres calculated from the two methods suggested that the pivoting point of the femur head during hip motion might not be at the center of femur head, and the former location seemed closer to the surface of head at the weight bearing site. Under such circumstances, the mechanical axis obtained through circumduction of the thigh might be 1∼2 degrees different from that obtained through the actual center of femur head. During knee flexion, the mechanical axis also changed gradually, and this could be due to laxity of knee joint, or due to intrinsic valgus/varus alignment. However, the value became unreliable when the knee was at a flexion angle of 60∼120 degrees, and this should be taken into account during navigation surgery

Introduction: The main purpose of this study was to analyze the accuracy of conventional versus navigated open wedge corrective osteotomies of the proximal tibia. Furthermore, the intraoperative radiation dosage and the time of the operative procedure of both groups were compared. Methods: 20 legs of 11 fresh cadaver (9 male, 2 female, age 35–71 years) were randomly assigned to conventional open wedge high tibial osteotomy (HTO) (n=10) or navigated open wedge HTO (n=10). Two legs had to be excluded because of pre-existing knee injuries. The aim of all corrective operations was to align the mechanical axis to pass through 80% of the tibial plateau (80% Fujisawa line), regardless of the preexisting alignment. The intraoperative mechanical axis was evaluated either by the cable technique for conventional HTO, or by a navigation module for navigated HTO (Medivision, Oberdorf/Switzerland). An angle fixed implant with interlocking screws (Tomofix, Mathys, Bettlach/Switzerland) was used to minimize postoperative loss of correction. Postoperatively, CT-scans were performed and the Fujisawaline and MPTA measured with a computer software for deformity analysis (Med-iCAD) The main outcome parameter was the accuracy of the correction, which was measured by the Fujisawa line. Secondary outcome parameters were the intraoperative radiation measured by the dose area product and the time of the operative procedure. For statistical analysis the standard deviation (S.D.) was calculated and the paired t-test applied. Results: After conventional HTO, the mechanical axis was intersecting the Fujisawa line at 72.1% of the tibial plateau (range 60.4–82.4%, S.D. 7.2%). In contrast, after navigated HTO the tibia plateau was passed through 79.7% (range 75.5–85.8%, S.D. 3.3%). Thus, the accuracy of the correction was significantly higher after navigated HTO (p=0.020). In addition, the standard deviation of the corrections was significantly lower after navigated HTO (p=0.012). The medial proximal tibia angle (MPTA) increased 7.9° (range: 4.7–12.1°) after conventional HTO and 9.1° (range: 4.6–12.6°) after navigated HTO. The average dose area products of the conventional HTO (49.5 cGy/cm2, range 36.0–81.2 cGy/cm2) and navigated HTO (42.8 cGy/cm2, range 28.3–58.1 cGy/cm2) were comparable (p=0.231). However, navigated HTO elongated the operation time significantly (navigated HTO: 82 min, range 55–98 min; conventional HTO: 59 min, range 47–73 min) (p< 0.001). Conclusion: Continuous three-dimensional imaging of the axis and of intraoperative tools with the a navigation module significantly improves the accuracy of open wedge osteotomies of the proximal tibia. Prospective clinical studies will show whether the results of this cadaver study can be transferred to the regular clinical use

Introduction:

The insertion footprint of the different muscles tendon fascicles of the Achilles Tendon on the calcanium tuberosity has not been described before.

Hypothesis. Recurrent shoulder dislocation is associated with bony defect of the glenoid rim, commonly seen along with bankart tear - a soft tissue injury of glenoid labrum. This cadaveric study compares the bone block effect of coracoid transfer using using two common techniques, Classical Latarjet technique and the Congruent-Arc Latarjet. We hypothesized that the force needed to dislocate the shoulder would be greater in Congruent Arc technique than the Classical Latarjet, because of increased contact surface area as a result of greater linear dimensions. Material and methods. We dissected 14 cadaveric shoulders. A bony Bankart lesion was created in form of an inverted pear glenoid. The humeral head was attached to a pulley system that was sequentially loaded until the shoulder dislocated anteriorly. The force needed to dislocate was noted. This was repeated after coracoid transfer with two common techniques, Classical Latarjet technique and the Congruent-Arc Latarjet. Results. The mean force required to dislocate shoulder post-Classical Latarjet technique was 325.71N, compared to 123.57 N in uncorrected shoulder. Similarly, the mean force required to dislocate shoulder post Congruent-Arc Latarjet technique was 327.14 N compared to 123.57 N in uncorrected shoulder. The two-tailed P value in either case was less than 0.0001, thus statistically significant. Unpaired t-test was done to compare the force required to dislocate the shoulder post procedure. Mean force required to dislocate shoulder post-Classical Latarjet, was 325.7N compared to 327N in post-Congruent Arc. The two-tailed P value equals 0.9020 and the 95% confidence interval was from −25.05 to 22.19, thus the difference was not statistically significant. Conclusion. The results confirm that both (Classical and Congruent-Arc Latarjet) techniques are good for addressing the shoulder instability, however bone block effect provided by one is not superior to other

Introduction

Roentgen stereophotogrammetric analysis (RSA) is currently the gold standard to measure early prosthetic migration which can predict aseptic loosening. However, RSA has some limitations such as the need for perioperative placed markers and exposure to X-radiation during follow up. Therefore, this study evaluates if low field MRI could be an alternative for RSA. Low field MRI was chosen because it is less hampered by metal artifacts of the prosthesis than high field MRI.

Implant alignment in knee arthroplasty has been identified as critical factor for a successful outcome. Human error during the registration process for imageless computer navigation knee arthroplasty directly affects component alignment. This cadaveric study aims to define the error in the registration of the landmarks and the resulting error in component alignment. Five fresh frozen cadaveric limbs including the hemipelvis were used for the study. Five surgeons performed the registration process via a medial parapatellar approach five times. In order to identify the gold standard point, the soft tissues were stripped and the registration was repeated by the senior author. Errors are presented as mm or degrees from the gold standard registration. The error range in the registration of the femoral centre in the coronal plane was 6.5mm laterally to 5.0mm medially (mean: −0.1, SD: 2.7). This resulted in a mechanical axis error of 5.2 degrees valgus to 2.9 degrees varus (mean: 0.1, SD: 1.1). In the sagittal plane this error was between −1.8 degrees (extension) and 2.7 degrees (flexion). The error in the calculation of the tibial mechanical axis ranged from −1.0 (valgus) to 2.3 (varus) degrees in the coronal plane and −3.2 degrees of extension to 1.3 degrees of flexion. Finally the error in calculating the transepicondylar axis was −11.2 to 6.3 degrees of internal rotation (mean: −3.2, SD: 3.9). The error in the registration process of the anatomical landmarks can result in significant malalignment of the components. The error range for the mechanical axis of the femur alone can exceed the 3 degree margin that has been previously been associated with implant longevity. The technique during the registration process is of paramount importance for image free computer navigation. Future research should be directed towards simplifying this process and minimizing the effect of human error

Introduction: The purpose of this cadaver study was to test feasibility and safety of a new technique for harvesting the FDL tendon through a plantar incision placed directly overlying the FDL division and to define the relevant surgical anatomy. Materials and Methods: In eight cadaver feet the FDL tendon was exposed in the midfoot through a plantar incision. The FDL tendon was divided and pulled proximally through a wound in the hindfoot. All the tissues superficial to the FDL tendon were then reflected to check for any inadvertent damage to adjacent neurovascular structures. Results: The FDL division lies midway between the back of the heel and the base of the second toe and about 3.7 cm medial to the lateral border of the foot. The medial and the lateral plantar neurovascular bundles are respectively about 0.43 cm and 0.86 cm away from the FDL division. Conclusions: The FDL tendon can be harvested through a plantar incision. The adjacent neurovascular structures remained undamaged. Plantar surface anatomy guides placement of the plantar incision so that the incision can overlie directly over the FDL division

Abstract