Anthony M. DiGioia

Ultrasound Registration of the Bone Surface for Surgical Navigation

Objective: To allow non-invasive registration of the bone surface for computer-assisted surgery (CAS), this investigation reports the development and evaluation of intraoperative registration using 2D ultrasound (US) images. This approach employs automatic segmentation of the bone surface reflection from US images tagged with the 3D position to enable the application of CAS to minimally invasive procedures. Methods: The US-based registration method was evaluated in comparison to point-based registration, which is the predominant method in current clinical use. The absolute accuracy of the US-based registration was determined using a phantom pelvis, with fiducial registration providing the ground truth. The relative accuracy was determined by an intraoperative study comparing the US registration to the point-based registration obtained as part of the HipNav™ experimental protocol. Results: The phantom pelvis study demonstrated equivalent accuracy between point-and US-based registration under in vitro conditions. In the intraoperative study, the US-based registration was sufficiently consistent with the point-based registration to warrant larger-scale clinical trials of this non-invasive registration method. Conclusion: Ultrasound-based registration eliminates the need for physical contact with the bone surface as in point-based registration. As a result, non-invasive registration could fully unlock the potential of computer-assisted surgery, enabling development of the next generation of minimally invasive surgical procedures.

An image overlay system for medical data visualization.

Abstract Image Overlay is a computer display technique which superimposes computer images over the user’s direct view of the real world. The images are transformed in real-time so they appear to the user to be an integral part of the surrounding environment. By using Image Overlay with three-dimensional medical images such as CT reconstructions, a surgeon can visualize the data ‘in-vivo’, exactly positioned within the patient’s anatomy, and potentially enhance the surgeon’s ability to perform a complex procedure. This paper describes prototype Image Overlay systems and initial experimental results from those systems.

Functional pelvic orientation measured from lateral standing and sitting radiographs.

We prospectively obtained preoperative and 3-month postoperative lateral pelvic radiographs in the standing and sitting positions from 84 patients who underwent total hip arthroplasty. We measured pelvic orientation (flexion extension) using the anterior pelvic plane as defined by the anterior superior iliac spines and pubic tubercles as references. There was a trend towards upright pelvic alignment when standing, with a mean anterior pelvic plane angle of 1.2° (range, −22°-+27°). In the sitting position the pelvis tended to extend posteriorly, with a mean anterior pelvic plane angle of −36.2° (range, −64°-+4°). There was a wide variation in the arc of pelvic flexion extension as patients moved from standing to sitting, with arc of pelvic motion in some patients as mobile as 70° and in others as stiff as 5°. There was no significant variation between males and females or between preoperative and postoperative pelvic flexion extension. There were substantial variations in pelvic orientation when comparing standing and sitting for an individual patient and between different patients. This variation can be unpredictable, and may influence implant alignment and stability after total hip arthroplasty.Level of Evidence: Level II, diagnostic study. See the Guidelines for Authors for a complete description of levels of evidence.

A contact-coupled finite element analysis of the natural adult hip.

A non-linear two-dimensional finite element model was used to study phenomena of stress redistribution in the natural adult hip resulting from parametric material property variations in the juxtarticular regions of the femoral head. Despite the geometrical simplifications employed, the intra-articular contact stresses (computed using the FEAP program) were found to be in reasonable qualitative agreement with previous in vitro data for the case of a normal hip. Generalized sclerotic changes in the subchondral plate, as reflected either in apparent modulus increases or in plate thickening, were found to have only minor effects on the computed contact stress distribution, although stress levels within the plate itself were markedly influenced. Localized subchondral plate sclerosis, by contrast, led to marked stress elevations in the cartilage immediately overlying the stiffened bone. Cartilage modulus increases caused increased load uptake for a given imposed deformation, but involved stress distribution increases which were very nearly linearly proportional to the increases in resultant load magnitude. Friction coefficient elevations had no noticeable effects on normal contact stress or upon overall load transmission, but involved complex, possibly slip-related, changes in intra-articular and cartilaginous shear stresses.

Cup alignment error model for total hip arthroplasty.

Almost all computer-assisted orthopaedic surgery systems that rely on the anterior pelvic plane definition, such as in computed tomography and magnetic resonance image-based, fluoroscopy-based, and nonimage total hip replacement approaches, are derived from identifying two pairs of pelvic bony landmarks: anterior superior iliac spines and the pubic tubercles. Although these systems strive to achieve cup alignment accuracy of approximately 1°, even a minor failure to correctly identify these anatomic landmarks can lead to higher inaccuracies in the final cup alignment. This study shows how to examine the effects of these inaccuracies on the final acetabular cup implant orientation during total hip replacement by generating a kinematic model, which then is simulated. Simulation results indicate that, for example, a total error of 4 mm in measuring the anterior superior iliac spine and the pubic tubercles would result in a final cup orientation of 47° and 27° in abduction and version respectively, resulting in a 2° abduction error and 7° error in version when targeting 45° abduction and 20° version results. These calculations can be repeated for any error values.

A Balance Exercise Program Appears to Improve Function for Patients With Total Knee Arthroplasty: A Randomized Clinical Trial

Background Patients with total knee arthroplasty (TKA) have impaired balance and movement control. Exercise interventions have not targeted these impairments in this population. Objectives The purposes of this study were: (1) to determine the feasibility of applying a balance exercise program in patients with TKA, (2) to investigate whether a functional training (FT) program supplemented with a balance exercise program (FT+B program) could improve physical function compared with an FT program alone in a small group of individuals with TKA, and (3) to test the methods and calculate sample size for a future randomized trial with a larger study sample. Design This study was a double-blind, pilot randomized clinical trial. Setting The study was conducted in the clinical laboratory of an academic center. Participants The participants were 43 individuals (30 female, 13 male; mean age=68 years, SD=8) who underwent TKA 2 to 6 months prior to the study. Interventions The interventions were 6 weeks (12 sessions) of a supervised FT or FT+B program, followed by a 4-month home exercise program. Measurements Feasibility measures included pain, stiffness, adherence, and attrition. The primary outcome measure was a battery of physical performance tests: self-selected gait speed, chair rise test, and single-leg stance time. Secondary outcome measures were the Western Ontario and McMaster Universities Osteoarthritis Index and the Lower Extremity Functional Scale. Results Feasibility of the balance training in people with TKA was supported by high exercise adherence, a relatively low dropout rate, and no adverse events. Both groups demonstrated clinically important improvements in lower-extremity functional status. The degree of improvement seemed higher for gait speed, single-leg stance time, and stiffness in the FT+B group compared with the FT group. Limitations Due to the pilot nature of the study, differences between groups did not have adequate power to show statistical significance. Conclusions There is a need for conducting a larger randomized controlled trial to test the effectiveness of an FT+B program after TKA.

Computer-Assisted Orthopaedic Surgery

Over the last decade, orthopaedics has been one of the most active and diverse areas of development in computer-assisted surgery. Orthopaedics has been a specially challenging area of development as many orthopaedic surgeons consider the outcomes of conventional procedures generally successful. There is significant pressure to improve the efficiency and ergonomics, reduce cost and dependence on more expensive medical imaging modalities, and simplify the use and interfaces. Increasing emphasis on less invasive and minimally invasive procedures could give a significant boost to the adoption of computer-assisted surgery. In this paper, we present the overview of different approaches using a classification scheme that relies on two important criteria: 1) the autonomy of clinical action permitted to the system and 2) the imaging requirements

Periprosthetic fractures of the femur after total knee arthroplasty. A literature review and treatment algorithm.

Supracondylar femur fractures after total knee arthroplasty can significantly alter the quality of knee arthroplasties and provide a challenging problem for the treating surgeon. A review of the literatur and an approach to the treatment of these periprosthetic fractures is presented. The predisposing factors, mechanisms of injuries, and characteristics of the fractures are identified. A classification system is proposed based on a modified Neer grading system, the degree of comminution, and the location and character of the fracture. A treatment algorithm is developed and based on acceptable alignments of less than 5 mm translations, angulations less than 5°-10°, minimal rotation, less than 1 cm of femoral shortening, and proper tibiofemoral prosthetic joint alignment.

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 .

Measurements of acetabular cup position and pelvic spatial orientation after total hip arthroplasty using computed tomography/radiography matching

This study presents a clinical validation of postoperative measurements of acetabular cup alignment following total hip arthroplasty (THA). The methodology was based on concurrent anatomic three-dimensional (3D) measurements of both the acetabular cup alignment and pelvic orientation, using an original CT/X-ray matching algorithm named Xalign. The subjects were 19 patients who had undergone bilateral THA using CT-based surgical navigation. All patients had postoperative pelvic CT scans and multiple antero-posterior (AP) pelvic X-rays. Using a proprietary software algorithm, the X-rays included in the study were matched with the corresponding postoperative CT scans. The goal of this method was to allow 3D anatomic pelvic and acetabular measurements on two-dimensional AP X-rays. The postoperative cup abduction, version and pelvic flexion angles were determined in three different ways: using CT images directly, applying the Xalign method, and finally by performing conventional (abduction only) measurements on AP pelvic X-rays. The cup orientation measured on CT images was taken as the ground truth. The Xalign measurement errors were defined as the difference between the CT cup values and those obtained by applying the matching method. The mean cup abduction error was 0.85° ± 1.3° (± standard deviation) and the mean version error was 0.01° ± 1.99°. Conventionally measured cup abduction ranged from 44° to 62° and correlated significantly (p = 0.001, r = −0.5) with pelvic flexion angle, proving the linear negative correlation between pelvic flexion and the error in conventional radiographic cup measurements. The Xalign method offered reasonable accuracy for cup orientation, and allowed cup and pelvic 3D anatomic measurements at different times.