Using a calliper to restore the centre of the femoral head during total hip replacement.

Restoration of leg length and offset is an important goal in total hip replacement. This paper reports a calliper-based technique to help achieve these goals by restoring the location of the centre of the femoral head. This was validated first by using a co-ordinate measuring machine to see how closely the calliper technique could record and restore the centre of the femoral head when simulating hip replacement on Sawbone femur, and secondly by using CT in patients undergoing hip replacement. Results from the co-ordinate measuring machine showed that the centre of the femoral head was predicted by the calliper to within 4.3 mm for offset (mean 1.6 (95% confidence interval (CI) 0.4 to 2.8)) and 2.4 mm for vertical height (mean -0.6 (95% CI -1.4 to 0.2)). The CT scans showed that offset and vertical height were restored to within 8 mm (mean -1 (95% CI -2.1 to 0.6)) and -14 mm (mean 4 (95% CI 1.8 to 4.3)), respectively. Accurate assessment and restoration of the centre of the femoral head is feasible with a calliper. It is quick, inexpensive, simple to use and can be applied to any design of femoral component.

Assessment of a fictitious domain method for patient-specific biomechanical modelling of press-fit orthopaedic implantation.

In this article, we discuss an application of a fictitious domain method to the numerical simulation of the mechanical process induced by press-fitting cementless femoral implants in total hip replacement surgeries. Here, the primary goal is to demonstrate the feasibility of the method and its advantages over competing numerical methods for a wide range of applications for which the primary input originates from computed tomography-, magnetic resonance imaging- or other regular-grid medical imaging data. For this class of problems, the fictitious domain method is a natural choice, because it avoids the segmentation, surface reconstruction and meshing phases required by unstructured geometry-conforming simulation methods. We consider the implantation of a press-fit femoral artificial prosthesis as a prototype problem for sketching the application path of the methodology. Of concern is the assessment of the robustness and speed of the methodology, for both factors are critical if one were to consider patient-specific modelling. To this end, we report numerical results that exhibit optimal convergence rates and thus shed a favourable light on the approach.

A full geometric and photometric calibration method for oblique-viewing endoscopes.

Oblique-viewing endoscopes (oblique scopes) are widely used in computer assisted surgeries. The viewing direction of an oblique scope can be changed by rotating the scope cylinder; this extends the field of view, but also makes the geometric calibration process more difficult. Although few calibration methods have yet been developed, calibration is critical for the application of augmented-reality technologies such as stereo vision to procedures involving oblique scopes. Moreover, to our knowledge, no photometric calibration method has yet been introduced for endoscopes, even though such calibration is important for illumination-based visualization techniques such as shape-from-shading. In this paper, we present a complete calibration process for oblique-viewing endoscopes, estimating both geometric and photometric properties. Experimental results demonstrate that our methods are practical and accurate.

HyBAR: hybrid bone-attached robot for joint arthroplasty.

BACKGROUND: A number of small bone-attached surgical robots have been introduced to overcome some disadvantages of large stand-alone surgical robots. In orthopaedics, increasing demand on minimally invasive joint replacement surgery has also been encouraging small surgical robot developments. Among various technical aspects of such an approach, optimal miniaturization that maintains structural strength for high speed bone removal was investigated. METHODS: By observing advantages and disadvantages from serial and parallel robot structures, a new hybrid kinematic configuration was designed for a bone-attached robot to perform precision bone removal for cutting the femoral implant cavity during patellofemoral joint arthroplasty surgery. A series of experimental tests were conducted in order to evaluate the performance of the new robot, especially with respect to accuracy of bone preparation. RESULTS: A miniaturized and rigidly-structured robot prototype was developed for minimally invasive bone-attached robotic surgery. A new minimally invasive modular clamping system was also introduced to enhance the robotic procedure. Foam and pig bone experimental results demonstrated a successful implementation of the new robot that eliminated a number of major design problems of a previous prototype. CONCLUSIONS: For small bone-attached surgical robots that utilize high speed orthopaedic tools, structural rigidity and clamping mechanism are major design issues. The new kinematic configuration using hinged prismatic joints enabled an effective miniaturization with good structural rigidity. Although minor problems still exist at the prototype stage, the new development would be a significant step towards the practical use of such a robot.

Femur statistical atlas construction based on two-level 3D non-rigid registration.

The statistical atlas is a 3D medical image analysis tool to enable more patient-oriented and efficient diagnosis. The atlas includes information on geometry and its variation across populations. The comparison with information from other patients is very useful for objective quantitative diagnosis. The statistical atlas can also be used to solve other challenging problems such as image segmentation. As a key to the construction of statistical atlases, 3D registration remains an important yet unsolved problem in the medical image field due to the geometrical complexity of anatomical shapes and the computational complexity arising from the enormous size of volume data. In this work we developed a two-level framework to efficiently solve 3D non-rigid registration, and applied the method to the problem of constructing statistical atlases of the femur. In contrast to a general multi-resolution framework, we employed an interpolation to propagate the matching instead of repeating the registration scheme in each resolution. The registration procedure is divided into two levels: a low-resolution solution to the correspondences and mapping of surface models using Chui and Rangarajan's thin-plate spline (TPS)-based algorithm, followed by an interpolation to achieve high-resolution matching. Next, principal component analysis (PCA) is used to build the statistical atlases. Experimental results show the shape variation learned from the atlases, and also demonstrate that our method significantly improves the efficiency of registration without decreasing the accuracy of the atlases.

The relationship of the orientation of the transverse acetabular ligament and acetabular labrum to the suggested safe zones of cup positioning in total hip arthroplasty.

BACKGROUND: There is a need to develop and validate a method for establishing cup orientation that is patient specific and independent of the anterior pelvic plane. It is our hypothesis that the transverse acetabular ligament and acetabular labrum can be used to do this. The objective of this study is to define the orientation of the plane formed by the transverse acetabular ligament and acetabular labrum and to examine whether these local landmarks lie within the limits of acceptance for cup positioning. METHODS: Twenty-five consecutive patients, who were being investigated for labral tears with a MRI arthrogram of the hip, were enrolled in this prospective study. The orientation of the transverse acetabular ligament-labrum plane was determined by manually selecting points on the transverse acetabular ligament and labrum. The best-fit plane through these points was determined and its operative orientation expressed with respect to a constructed pelvic coordinate system. RESULTS: The operative anteversion of the transverse acetabular ligament-labrum plane ranged from 5.3-36.1 inverted exclamation mark (mean 23.0 inverted exclamation mark + or - 7.4 inverted exclamation mark standard deviation). The inclination ranged from 38.4-50.3 inverted exclamation mark (mean 45.6 inverted exclamation mark + or - 3.2 inverted exclamation mark standard deviation). CONCLUSIONS: The transverse acetabular ligament and acetabular labrum offer a possible solution to the many difficulties involved in cup placement during total hip arthroplasty. This paper highlights the variation in the orientation of these local acetabular landmarks and questions the logic of a set target for cup positioning.

Variations in acetabular anatomy with reference to total hip replacement.

Three-dimensional surface models of the normal hemipelvis derived from volumetric CT data on 42 patients were used to determine the radius, depth and orientation of the native acetabulum. A sphere fitted to the lunate surface and a plane matched to the acetabular rim were used to calculate the radius, depth and anatomical orientation of the acetabulum. For the 22 females the mean acetabular abduction, anteversion, radius and normalised depth were 57.1 degrees (50.7 degrees to 66.8 degrees ), 24.1 degrees (14.0 degrees to 33.3 degrees ), 25 mm (21.7 to 30.3) and 0.79 mm (0.56 to 1.04), respectively. The same parameters for the 20 males were 55.5 degrees (47.7 degrees to 65.9 degrees ), 19.3 degrees (8.5 degrees to 32.3 degrees ), 26.7 mm (24.5 to 28.7) and 0.85 mm (0.65 to 0.99), respectively. The orientation of the native acetabulum did not match the safe zone for acetabular component placement described by Lewinnek. During total hip replacement surgeons should be aware that the average abduction angle of the native acetabulum exceeds that of the safe zone angle. If the concept of the safe zone angle is followed, abduction of the acetabular component should be less than the abduction of the native acetabulum by approximately 10 degrees .

Accuracy of ultrasound to MR registration of the knee.

BACKGROUND: Ultrasound-based registration to 3D surfaces segmented from MR imaging is proposed as a non-invasive alternative to point-based registration for image-guided surgery. By relying upon diagnostic MR imaging, the expense of additional CT imaging (and exposure to radiation) is avoided. The technique would enable navigation in arthroscopic and other minimally invasive procedures. METHODS: Optically tracked registrations using point-based and ultrasound-based methods to MR and CT imaging volumes for two cadaveric specimens were acquired and analysed. RESULTS: The average RMS distance between fiducials was 0.27 mm for CT and 0.72 mm for MR utilizing point-based registration. The average RMS distance for ultrasound-based registration to CT was 0.59 mm and 0.76 mm to MR. The RMS distance for fiducials co-located in MR and CT imaging volumes was 0.74 mm. The end-to-end error of ultrasound registration to MR imaging was 2.98 mm, as compared to 1.65 mm for CT. CONCLUSIONS: Ultrasound registration to MR imaging data is a viable non-invasive alternative to point-based registration.

MBARS: mini bone-attached robotic system for joint arthroplasty.

A report on a new active, miniature bone-attached, robotic system including its design, high level and low level control, is given together with a description of the system implementation and first experimental use. The system is capable of preparing the bone cavity for an implant during joint arthroplasty procedures. Without loss of generality, the report describes the implementation of the system for a Patellofemoral Joint Replacement procedure. The system is image-free and all planning is performed intra-operatively in the robot coordinate system, eliminating the need for external tracking systems in the operating room. Experiments were conducted using the first robot prototype and the results supported the feasibility of the concept. The methodology which is presented can be modified to other orthopaedic procedures and could improve the results in terms of accuracy and operational time. Moreover, it enables minimally invasive procedures and use of the next generation of more anatomically shaped implants.

Accuracy in tunnel placement for ACL reconstruction. Comparison of traditional arthroscopic and computer-assisted navigation techniques.

The purpose of this randomized, prospective study was to compare accuracy in tunnel placement as performed with a traditional arthroscopic anterior cruciate ligament (ACL) reconstruction technique and with KneeNavTM ACL, a computer-assisted surgical navigation technique. Two surgeons experienced in ACL reconstruction, but inexperienced in computer-assisted surgical navigation technique, each randomly used traditional arthroscopic guides or KneeNavTM ACL to drill a tunnel in twenty identical foam knees. Placement of the resulting tibial and femoral tunnels was measured with a computer-assisted digitizing method and compared to traditional biplanar radiographs. Statistical analysis with Student's t-test was used to compare the distance from the ideal tunnel placement to the femoral and tibial tunnels. Accuracy of tunnel placement with KneeNavTM ACL was significantly better than that obtained with the traditional arthroscopic technique. Distances from the ideal tunnel placement to the femoral and tibial tunnels were 4.2 +/- 1.8 mm (mean +/- SD) and 4.9 +/- 2.3 mm, respectively, for the traditional arthroscopic technique, and 2.7 +/- 1.9 mm (femur) and 3.4 +/- 2.3 mm (tibia) for KneeNavTM ACL. These differences were statistically different. Tunnel placement for ACL reconstruction with KneeNavTM ACL, an image-based, computer-assisted surgical navigation device with a simple and intuitive interface, was more accurate than with the traditional arthroscopic technique.

Computer assisted orthopaedic surgery. Image guided and robotic assistive technologies.

Technologies are emerging that will influence the way in which orthopaedic surgery is planned, simulated, and performed. Recent advances in the fields of medical imaging, computer vision, and robotics have provided the enabling technologies to permit computer aided surgery to become an established area which can address clinical needs. Although these technologies have been applied in industry for more than 20 years, the field of computer assisted orthopaedic surgery is still in its infancy. Image guided and surgical navigation systems, robotic assistive devices, and surgical simulators have begun to emerge from the laboratory and hold the potential to improve current surgical practice and patients' outcomes. The goals of these new clinically focused technologies are to develop interactive, patient specific preoperative planners to optimize the performance of surgery and the postoperative biologic response, and develop more precise and less invasive interactive smart tools and sensors to assist in the accurate and precise performance of surgery. The medical community is beginning to see the benefit of these enabling technologies which can be realized only through the collaboration and combined expertise of engineers, roboticists, computer scientists, and surgeons.

Computer assisted measurement of cup placement in total hip replacement.

The introduction of image guided systems in total hip replacement surgery provides the ability to plan precisely the alignment of the acetabular cup before surgery, and to perform the surgery according to the preoperative plan. Preoperative planners (interactive computer programs for surgical planning) based on three-dimensional medical images allow planning of optimal placement of implant components based on simulated implant performance. Exact measurement of the cup position during surgery allows precise placement of the cup and accurate measurement of the final position of the cup relative to the pelvis. This measurement is used to evaluate the radiographic techniques for postoperative measurement of cup alignment. Malposition of the acetabular component increases the occurrence of impingement, reduces the safe range of motion, and increases the risk of dislocation and wear. Dislocation of the implant after total hip replacement remains a significant clinical problem. Not fully understanding the interaction between pelvic orientation and final acetabular cup alignment may be one of the main contributing factors in the continued significant incidence of dislocations after total hip replacement. In this study an attempt was made to link the preoperative planning, intraoperative placement, and postoperative measurement of cup placement in total hip replacement using computer assisted techniques.

The Otto Aufranc Award. Image guided navigation system to measure intraoperatively acetabular implant alignment.

There has been little clinical research to examine the effects of patient positioning and pelvic motion on the alignment of the acetabular implant during total hip replacement surgery. Until now, no tools were capable of accurately measuring these variables during the actual procedure. As part of a broader program in medical robotics and computer assisted surgery, a clinical system has been developed that includes several enabling technologies. The hip navigation system (HipNav) continuously and precisely measures pelvic location and tracks relative implant alignment intraoperatively. HipNav technology is used to gauge current clinical practice and provide intraoperative feedback to surgeons with the goal of improving the precision and accuracy of acetabular alignment during total hip replacement. This system provides surgeons with a new class of image guided measurement tools and assist devices. These tools successfully were introduced into the clinical practice of surgery with results showing the following: (1) There exist unpredictable and large variations in the initial position of patients' pelves on the operating room table and significant pelvic movement during surgery and during intraoperative range of motion testing; (2) current mechanical acetabular alignment guides do not account for these variations, and result in variable and in the majority of cases unacceptable acetabular alignment; and (3) press fitting oversized acetabular components influences the final cup orientation.

Biomechanics for preoperative planning and surgical simulations in orthopaedics.

Surgical simulations are particularly appropriate for the large volume and expense of joint replacement procedures in orthopaedics. A first generation surgical simulator has been developed to model the implantation procedure for cementless acetabular and femoral components in total hip replacement surgery. The simulator is based upon finite element analysis and predicts the early postoperative mechanical environment that results from a proposed surgery. Since the short- and long-term clinical success of cementless hip replacement components is very dependent upon the initial mechanics of the bone-implant system, such simulations can help orthopaedic surgeons to develop better preoperative plans.