Jyri Lepistö

Computed tomography measurement of acetabular dimensions: Normal values for correction of dysplasia

Background A successful periacetabular osteotomy includes reliable planning based on radiographs and CT scanning. However, we lack normative CT values in planning realignment of the osteotomized acetabulum. Patients and methods We retrospectively studied 70 hips that had been CT-scanned. Patients who showed no signs of developmental disturbances in either of the hip joints were eligible for the study. Sex differences were also studied. Results The AA-angle, CE-angle, ACE-angle and AcetAV-angle, depicting frontal, sagittal and horizontal alignment, averaged 3° (SD 4°), 41° (7°), 31° (5°) and 21° (7°), respectively. The upper normal value (+ 2SD) for the AA-angle was 12°, normal range (± 2SD) for CE-angle was 27°–55°, lower normal value (– 2SD) for the ACE-angle was 22°, and normal range (± 2SD) for the AcetAV-angle was 6°–35°. However, comparison of mean angles in women with those in men showed a statistically significant difference for the AA-angle and AcetAV-angle, but we found no significant differences between the mean figures for right and left hips. Interpretation Knowledge of the normal dimensions of the acetabulum is essential in the diagnosis of the type and severity of DDH, as well as in preoperative planning. Accurate estimation of the normal contact surface orientation permits correct realignment of the osteotomized acetabulum

Three-dimensional mechanical evaluation of joint contact pressure in 12 periacetabular osteotomy patients with 10-year follow-up

Background and purpose Because of the varying structure of dysplastic hips, the optimal realignment of the joint during periacetabular osteotomy (PAO) may differ between patients. Three-dimensional (3D) mechanical and radiological analysis possibly accounts better for patient-specific morphology, and may improve and automate optimal joint realignment. Patients and methods We evaluated the 10-year outcomes of 12 patients following PAO. We compared 3D mechanical analysis results to both radiological and clinical measurements. A 3D discrete-element analysis algorithm was used to calculate the pre- and postoperative contact pressure profile within the hip. Radiological angles describing the coverage of the joint were measured using a computerized approach at actual and theoretical orientations of the acetabular cup. Quantitative results were compared using postoperative clinical evaluation scores (Harris score), and patient-completed outcome surveys (q-score) done at 2 and 10 years. Results The 3D mechanical analysis indicated that peak joint contact pressure was reduced by an average factor of 1.7 subsequent to PAO. Lateral coverage of the femoral head increased in all patients; however, it did not proportionally reduce the maximum contact pressure and, in 1 case, the pressure increased. This patient had the lowest 10-year q-score (70 out of 100) of the cohort. Another hip was converted to hip arthroplasty after 3 years because of increasing osteoarthritis. Interpretation The 3D analysis showed that a reduction in contact pressure was theoretically possible for all patients in this cohort, but this could not be achieved in every case during surgery. While intraoperative factors may affect the actual surgical outcome, the results show that 3D contact pressure analysis is consistent with traditional PAO planning techniques (more so than 2D analysis) and may be a valuable addition to preoperative planning and intraoperative assessment of joint realignment.

Outcome of periacetabular osteotomy: Joint contact pressure calculation using standing AP radiographs, 12 patients followed for average 2 years

Background Due to wide variations in acetabular structure of individuals with hip dysplasia, the measurement of the acetabular orientation may not be sufficient to predict the joint loading and pressure distribution across the joint. Addition of mechanical analysis to preoperative planning, therefore, has the potential to improve the clinical outcome. We analyzed the effect of periacetabular osteotomy on hip dysplasia using computeraided simulation of joint contact pressure on regular AP radiographs. The results were compared with the results of surgery based on realignment of acetabular angles to the normal hip. Patients and methods We studied 12 consecutive periacetabular osteotomies with no femoral head deformity. The median age of patients, all females, was 35 (20–50) years. The median follow-up was 2 years (1.3–2.2). Patient outcome was measured with the total score of a self-administered questionnaire (q-score) and with the Harris hip score. The pre- and postoperative orientation of the acetabulum was defined using reconstructed 3D CT-slices to measure angles in the three anatomical planes. Peak contact pressure, weight-bearing area, and the centroid of the contact pressure distribution (CP-ratio) were calculated. Results While 9 of 12 cases showed decreased peak pressure after surgery, the mean changes in weight-bearing area and peak contact pressure were not statistically significant. However, CP-ratio changed (p < 0.001, paired t-test) with surgery. For the optimal range of CP-ratio (within its mid-range 40–60%), the mechanical outcome improved significantly. Interpretation Verifying the correlation between the optimal CP-ratio and the outcome of the surgery requires additional studies on more patients. Moreover, the anatomically measured angles were not correlated with the ranges of CP-ratio, suggesting that they do not always associate with objective mechanical goals of realignment osteotomy. Mechanical analysis, therefore, can be a valuable tool in assessing two-dimensional radiographs in hip dysplasia.

Evaluation of a computerized measurement technique for joint alignment before and during periacetabular osteotomy

Periacetabular osteotomy (PAO) is intended to treat a painful dysplastic hip. Manual radiological angle measurements are used to diagnose dysplasia and to define regions of insufficient femoral head coverage for planning PAO. No method has yet been described that recalculates radiological angles as the acetabular bone fragment is reoriented. In this study, we propose a technique for computationally measuring the radiological angles from a joint contact surface model segmented from CT-scan data. Using oblique image slices, we selected the lateral and medial edge of the acetabulum lunate to form a closed, continuous, 3D curve. The joint surface is generated by interpolating the curve, and the radiological angles are measured directly using the 3D surface. This technique was evaluated using CT data for both normal and dysplastic hips. Manual measurements made by three independent observers showed minor discrepancies between the manual observations and the computerized technique. Inter-observer error (mean difference +/- standard deviation) was 0.04 +/- 3.53 degrees for Observer 1; -0.46 +/- 3.13 degrees for Observer 2; and 0.42 +/- 2.73 degrees for Observer 3. The measurement error for the proposed computer method was -1.30 +/- 3.30 degrees . The computerized technique demonstrates sufficient accuracy compared to manual techniques, making it suitable for planning and intraoperative evaluation of radiological metrics for periacetabular osteotomy.

Biomechanical factors in planning of periacetabular osteotomy

Objective: This study addresses the effects of cartilage thickness distribution and compressive properties in the context of optimal alignment planning for periacetabular osteotomy (PAO). Background: The Biomechanical Guidance System (BGS) is a computer-assisted surgical suite assisting surgeon’s in determining the most beneficial new alignment of a patient’s acetabulum. The BGS uses biomechanical analysis of the hip to find this optimal alignment. Articular cartilage is an essential component of this analysis and its physical properties can affect contact pressure outcomes. Methods: Patient-specific hip joint models created from CT scans of a cohort of 29 dysplastic subjects were tested with four different cartilage thickness profiles (one uniform and three non-uniform) and two sets of compressive characteristics. For each combination of thickness distribution and compressive properties, the optimal alignment of the acetabulum was found; the resultant geometric and biomechanical characterization of the hip were compared among the optimal alignments. Results: There was an average decrease of 49.2 ± 22.27% in peak contact pressure from the preoperative to the optimal alignment over all patients. We observed an average increase of 19 ± 7.7° in center-edge angle and an average decrease of 19.5 ± 8.4° in acetabular index angle from the preoperative case to the optimized plan. The optimal alignment increased the lateral coverage of the femoral head and decreased the obliqueness of the acetabular roof in all patients. These anatomical observations were independent of the choice for either cartilage thickness profile, or compressive properties. Conclusion: While patient-specific acetabular morphology is essential for surgeons in planning PAO, the predicted optimal alignment of the acetabulum was not significantly sensitive to the choice of cartilage thickness distribution over the acetabulum. However, in all groups the biomechanically predicted optimal alignment resulted in decreased joint contact pressure and improved acetabular coverage.

Statistical atlas based extrapolation of CT data

We present a framework to estimate the missing anatomical details from a partial CT scan with the help of statistical shape models. The motivating application is periacetabular osteotomy (PAO), a technique for treating developmental hip dysplasia, an abnormal condition of the hip socket that, if untreated, may lead to osteoarthritis. The common goals of PAO are to reduce pain, joint subluxation and improve contact pressure distribution by increasing the coverage of the femoral head by the hip socket. While current diagnosis and planning is based on radiological measurements, because of significant structural variations in dysplastic hips, a computer-assisted geometrical and biomechanical planning based on CT data is desirable to help the surgeon achieve optimal joint realignments. Most of the patients undergoing PAO are young females, hence it is usually desirable to minimize the radiation dose by scanning only the joint portion of the hip anatomy. These partial scans, however, do not provide enough information for biomechanical analysis due to missing iliac region. A statistical shape model of full pelvis anatomy is constructed from a database of CT scans. The partial volume is first aligned with the statistical atlas using an iterative affine registration, followed by a deformable registration step and the missing information is inferred from the atlas. The atlas inferences are further enhanced by the use of X-ray images of the patient, which are very common in an osteotomy procedure. The proposed method is validated with a leave-one-out analysis method. Osteotomy cuts are simulated and the effect of atlas predicted models on the actual procedure is evaluated.

Clinical evaluation of a biomechanical guidance system for periacetabular osteotomy

BackgroundPopulations suffering from developmental dysplasia of the hip typically have reduced femoral coverage and experience joint pain while walking. Periacetabular osteotomy (PAO) is one surgical solution that realigns the acetabular fragment. This challenging surgery has a steep learning curve. Existing navigation systems for computer-assisted PAO neither track the released fragment nor offer the means to assess fragment location. An intraoperative workstation—the biomechanical guidance system (BGS)—developed for PAO incorporates intraoperative fragment tracking and acetabular characterization through radiographic angles and joint biomechanics. In this paper, we investigate the accuracy and effectiveness of the BGS for bone fragment tracking and acetabular characterization in clinical settings as compared to conventional techniques and postoperative assessments. We also report the issues encountered and our remedies when using the BGS in the clinical setting.MethodsEleven consecutive patients (aged 22–48, mean 34, years) underwent 12 PAO surgeries (one bilateral surgery) where the BGS collected information on acetabular positioning. These measurements were compared with postoperative CT data and manual measurements made intraoperatively.ResultsNo complications were reported during surgery, with surgical time—95–210 (mean 175) minutes—comparable to reported data for the conventional approach. The BGS-measured acetabular positioning showed strong agreement with postoperative CT measurements (−0.3–9.2, mean 3.7, degrees), whereas larger differences occurred between the surgeon’s intraoperative manual measurements and postoperative CT measurements (−2.8–21.3, mean 10.5, degrees).ConclusionsThe BGS successfully tracked the acetabular fragment in a clinical environment without introducing complications to the surgical workflow. Accurate 3D positioning of the acetabulum may provide more information intraoperatively (e.g., anatomical angles and biomechanics) without adversely impacting the surgery to better understand potential patient outcomes.

Biomechanical Guidance System for Periacetabular Osteotomy

This chapter presents a biomechanical guidance navigation system for performing periacetabular osteotomy (PAO) to treat developmental dysplasia of the hip. The main motivation of the biomechanical guidance system (BGS) is to plan and track the osteotomized fragment in real time during PAO while simplifying this challenging procedure. The BGS computes the three-dimensional position of the osteotomized fragment in terms of conventional anatomical angles and simulates biomechanical states of the joint. This chapter describes the BGS structure and its application using two different navigation approaches including optical tracking of the fragment and x-ray-based navigation. Both cadaver studies and preliminary clinical studies showed that the biomechanical planning is consistent with traditional PAO planning techniques and that the additional information provided by accurate 3D positioning of the fragment does not adversely impact the surgery.

Reliability of computer-assisted periacetabular osteotomy using a minimally invasive approach

BackgroundPeriacetabular osteotomy (PAO) is the treatment of choice for younger patients with developmental hip dysplasia. The procedure aims to normalize the joint configuration, reduce the peak-pressure, and delay the development of osteoarthritis. The procedure is technically demanding and no previous study has validated the use of computer navigation with a minimally invasive transsartorial approach.MethodsComputer-assisted PAO was performed on ten patients. Patients underwent pre- and postoperative computed tomography (CT) scanning with a standardized protocol. Preoperative preparation consisted of outlining the lunate surface and segmenting the pelvis and femur from CT data. The Biomechanical Guidance System was used intra-operatively to automatically calculate diagnostic angles and peak-pressure measurements. Manual diagnostic angle measurements were performed based on pre- and postoperative CT. Differences in angle measurements were investigated with summary statistics, intraclass correlation coefficient, and Bland–Altman plots. The percentage postoperative change in peak-pressure was calculated.ResultsIntra-operative reported angle measurements show a good agreement with manual angle measurements with intraclass correlation coefficient between 0.94 and 0.98. Computer navigation reported angle measurements were significantly higher for the posterior sector angle (