Limitations of imageless computer-assisted navigation for total hip arthroplasty.

We prospectively evaluated acetabular cup placement in total hip arthroplasty with an imageless computer navigation system or using conventional manual technique. The achieved cup orientation in the manual group had substantially larger variances and greater placement error than the navigation cases. The use of navigation was abandoned in 3 cases because of excessive pelvic tilt and unreliable registration of the pelvis. Computer navigation system helped improve the accuracy of the acetabular cup placement for total hip arthroplasty in this study. The variation between the intraoperative navigation readings and the computed tomographic values suggests that relying on palpation of bony landmarks through drapes and tissue is a limitation of this method. Further, the variation in pelvic tilt has an effect on cup placement that requires further studies.

Use of MRI images to measure tissue thickness over the ischial tuberosity at different hip flexion.

The goal of this experiment was to investigate changes in the thickness of the soft tissue overlying the ischial tuberosity (IT) due to changes in hip flexion angle and the addition of a sitting load. Eleven healthy subjects were tested. An apparatus constructed from foam blocks and an air bladder was used to position the subjects in different postures within an MRI tube. MRI images of the buttocks and thigh were obtained for four postures: Supine, 45° Hip Flexion, Non-Weight-Bearing 90° Hip-Flexion, and Weight-Bearing 90° Hip-Flexion. The thickness of muscle, adipose tissue, and skin was measured between the IT tip and skin surface, perpendicular to the cushion placed beneath the thighs. The tissue overlying the IT was found to be significantly (P < 0.001) thinner in 90° Hip-Flexion (73.8 ± 9.0 mm) than in the supine position (135.9 ± 8.1 mm). Muscle thickness decreased significantly from Supine to Non-Weight-Bearing 90° Hip-Flexion (59.1 ± 8.5%, P < 0.001), and further decreased from Non-Weight-Bearing to Weight-Bearing 90° Hip-Flexion (46.2 ± 7.9%, P < 0.001). Under Weight-Bearing 90° Hip-Flexion, the muscle tissue deformed significantly (P < 0.001) more than the adipose tissue and skin. We concluded that the tissue thickness covering the IT significantly decreased with hip flexion, and further decreased by nearly half during loading caused by sitting. In addition, the muscle tissue experienced the largest deformation during sitting. The results of this study may improve our understanding of risk factors for pressure ulcer development due to changes in tissue padding over the IT in different postures.

Biomechanical effects of sitting with adjustable ischial and lumbar support on occupational low back pain: evaluation of sitting load and back muscle activity.

BACKGROUND: Compared to standing posture, sitting decreases lumbar lordosis, increases low back muscle activity, disc pressure, and pressure on the ischium, which are associated with occupational LBP. A sitting device that reduces spinal load and low back muscle activities may help increase sitting comfort and reduce LBP risk. The objective of this study is to investigate the biomechanical effect of sitting with a reduced ischial support and an enhanced lumbar support (Off-Loading) on load, interface pressure and muscle activities. METHODS: A laboratory test in low back pain (LBP) and asymptomatic subjects was designed to test the biomechanical effect of using the Off-Loading sitting posture. The load and interface pressure on seat and the backrest, and back muscle activities associated with usual and this Off-Loading posture were recorded and compared between the two postures. RESULTS: Compared with Normal (sitting upright with full support of the seat and flat backrest) posture, sitting in Off-Loading posture significantly shifted the center of the force and the peak pressure on the seat anteriorly towards the thighs. It also significantly decreased the contact area on the seat and increased that on the backrest. It decreased the lumbar muscle activities significantly. These effects are similar in individuals with and without LBP. CONCLUSION: Sitting with reduced ischial support and enhanced lumbar support resulted in reduced sitting load on the lumbar spine and reduced the lumbar muscular activity, which may potentially reduce sitting-related LBP.

Validation of a computer navigation system and a CT method for determination of the orientation of implanted acetabular cup in total hip arthroplasty: a cadaver study.

BACKGROUND: Successful hip reconstruction to restore the normal hip biomechanics requires precise placement of implants. Computer assisted navigation in total hip arthroplasty has been proposed to have the potential to help achieve a high accuracy in implant placement. The goal of the study was to evaluate the accuracy of an imageless computer navigation system on cadavers and to validate a non-invasive computed tomography method for post-operative determination of acetabular cup orientation. METHODS: Total hip arthroplasty was performed on seven cadaver hips with the aid of an imageless computer navigation system. The achieved cup orientation were recorded using three methods, (1) intra-operatively using the imageless computer navigation system, (2) post-operatively with direct bone digitization and (3) with a computed tomography based three dimensional model interpreted by three raters. Measurement from the direct bone digitization was taken as the gold standard to evaluate the other two methods. The intra-rater and inter-rater consistency of the computer tomography-model method were assessed by Cronbach's alpha determination. FINDINGS: Compared with the cup orientation obtained from the direct bone digitization, the average difference for anteversion and abduction were 3.3 (3.5) degrees (P=0.045) and 0.6 (3.7) degrees , respectively, for navigation reading. The average differences for computer tomography-model for three raters were 0.5 (2.1) degrees , 0.8 (1.5) degrees and 3.2 (3.3) degrees (P=0.043) for anteversion and 0.4 (1.6) degrees , 0.3 (1.6) degrees and 2.1 (2.7) degrees for abduction. The intra-rater consistency ranged from 0.626 for a novice rater to over 0.97 for experience raters. The inter-rater consistency (including novice and experienced raters) was over 0.90. INTERPRETATION: While the values for cup orientation determined with imageless computer navigation were comparable to those from direct bone and implant digitization, the measurement for anteversion obtained was not as accurate as that for abduction. The proposed computer tomography-model method has an excellent intra-rater consistency for experienced raters, as well as an excellent overall inter-rater consistency. The study confirms that a non-invasive computed tomography based model analysis can be used in clinical practice as a valid method for post-operatively evaluating the orientation of the acetabular component.

Evaluation of a new sitting concept designed for prevention of pressure ulcer on the buttock using finite element analysis.

Excessive compressive load induces pressure related soft tissue damage, i.e. pressure ulcer (PU), in buttock area in wheelchair users. In solving this problem, our previous study has introduced a concept of Off-Loading sitting, which partially removes the ischial support to reduce pressure under buttocks. However, the effect of this sitting concept has only been evaluated using the interface pressure and tissue perfusion measurements. The objective of this investigation was to evaluate the Off-Loading posture for its ability to reduce internal pressure and stress in deep buttock tissues. This evaluation was performed on a 3D finite element (FE) model which was established and validated in a sitting posture and has realistic material properties and boundary conditions. FE analysis in this study confirmed that the pressure relief provided by Off-Loading posture created profound effect in reducing the mechanical stress within deep tissues. It was concluded that Off-Loading posture may prove beneficial in preventing sitting related PU.

In vivo and noninvasive three-dimensional patellar tracking induced by individual heads of quadriceps.

PURPOSE: Unbalanced actions of the quadriceps components are closely linked to patellar mal-tracking and patellofemoral pain syndrome. However, it is not clear how individual quadriceps components pull and rotate the patella three dimensionally. The purpose of this study was to investigate in vivo and noninvasively patellar tracking induced by individual quadriceps components. METHODS: Individual quadriceps component was activated selectively through electrical stimulation at the muscle motor point, and the resulting patellar tracking was measured in vivo and noninvasively in 18 knees of 12 subjects. The in vivo and noninvasively patellar tracking was corroborated with in vivo fluoroscopy and in vitro cadaver measurements. RESULTS: Vastus medialis (VM) mainly pulled the patella first in the medial and second in the proximal directions and vastus lateralis (VL) pulled first in the proximal and second in the lateral directions. The oblique portion (VMO) of the VM pulled the patella mainly medially and the longus portion (VML) more proximally. Medial tilt was the major patellar rotation induced by VMO contraction at full knee extension. With the knee at the more flexed positions, the amplitude of patellar movement induced by comparable quadriceps contractions was reduced significantly compared to that at full knee extension, and VMO changed its main action from extending to flexing the patella. CONCLUSIONS: The medial and lateral quadriceps components moved the patella in rather different directions, and rotated the patella differently about the mediolateral tilt and mediolateral rotation axes but similarly in extension. The approach can be used to investigate patellar tracking in vivo and noninvasively in both healthy subjects and patients with patellofemoral disorder and patellar malalignment.

Use of helical computed tomography for the assessment of acetabular osteolysis after total hip arthroplasty.

BACKGROUND: Acetabular osteolysis is a major problem affecting long-term survival of total hip prostheses. Since lytic lesions may be asymptomatic until extensive bone loss has occurred, early detection of lytic lesions is important. The purposes of this study were to determine the efficacy and potential role of high-resolution helical (or spiral) computed tomography with metal-artifact minimization in the early detection of osteolysis of the pelvis and to use the method to determine if there was a relationship between the extent of osteolysis and the amount of polyethylene wear. METHODS: Forty patients (fifty hips) who had undergone primary cementless total hip arthroplasty between 1988 and 1994 were evaluated as part of an ongoing prospective study. These patients had a history of high-level activity that was believed to place them at increased risk for accelerated polyethylene wear. The most recent follow-up radiographs were compared with the three-month postoperative radiographs. Helical computed tomography scans with metal-artifact minimization were made, and evidence of osteolytic lesions on these scans was compared with that on the radiographs. Two-dimensional wear analysis was performed with use of digitized radiographs, and the results were compared with loss of bone volume as calculated from the computed tomography scans. RESULTS: Acetabular lysis was identified on the radiographs of sixteen hips and on the computed tomography scans of twenty-six hips. Radiographs underestimated the extent of the lysis in thirteen of the sixteen hips. There was no correlation (r = 0.036) between linear wear and the measured volume of bone loss, with the numbers available. On the basis of the amount of lysis seen on the computed tomography scans, one patient underwent a revision procedure. CONCLUSIONS: Helical computed tomography with metal-artifact minimization is more sensitive for identifying and quantifying osteolysis after total hip arthroplasty than is plain radiography. Since computed tomography scans show both the extent and the location of lytic lesions, they are useful to guide treatment decisions as well as to assist in planning for surgical intervention, when needed, in patients with suspected osteolysis.

In vivo and noninvasive six degrees of freedom patellar tracking during voluntary knee movement.

OBJECTIVE: The purpose of this study was to investigate in vivo and noninvasively patellar tracking in six degrees of freedom during voluntary knee extension and flexion. DESIGN: Patellar tracking was evaluated in vivo and noninvasively with corroboration using in vivo fluoroscopy and in vitro cadaver measurements. BACKGROUND: Patellofemoral pain is closely related to abnormal patellar tracking and malalignment. However, there is a lack of quantitative and convenient methods to evaluate six degrees of freedom in vivo patellar tracking, partly due to difficulty in evaluating 3-D patellar tracking noninvasively. METHODS: Six degrees of freedom patellar tracking was measured in vivo and noninvasively using a small clamp mounted onto the patella and an optoelectronic motion capture system in 18 knees of 12 healthy subjects during voluntary knee extension and flexion. RESULTS: The patella tracked systematically following a certain pattern during knee extension and flexion. Patellar tracking patterns during knee extension and flexion were not significantly different in the 18 knees tested. When the knee was voluntarily extended from 15 degrees flexion to full extension, the patella was extended 8 degrees, laterally tilted 2 degrees, and shifted 3 mm laterally and 10 mm proximally. The results were consistent with previous in vitro and in vivo studies. CONCLUSION: Six degrees of freedom patellar tracking can be evaluated in vivo and noninvasively within the range of 20 degrees flexion to full knee extension. RELEVANCE: The study provided us quantitative six degrees of freedom information about patellar tracking during knee flexion/extension, which can be used to investigate patellar tracking in vivo and noninvasively in both healthy subjects and patients with patellofemoral disorder and patellar malalignment.

In vivo noninvasive evaluation of abnormal patellar tracking during squatting in patients with patellofemoral pain.

BACKGROUND: Patellofemoral pain syndrome is one of the most common knee problems and may be related to abnormal patellar tracking. Our purpose was to compare, in vivo and noninvasively, the patellar tracking patterns in symptomatic patients with patellofemoral pain and those in healthy subjects during squatting. We tested the hypothesis that patients with patellofemoral pain exhibit characteristic patterns of patellar tracking that are different from those of healthy subjects. METHODS: Three-dimensional patellar kinematics were recorded in vivo with use of a custom-molded patellar clamp and an optoelectronic motion capture system in ten healthy subjects and nine subjects with patellofemoral pain. The position of osseous knee landmarks was digitized while subjects stood upright, and then patellofemoral kinematics were recorded during squatting. The tracking technique was validated with use of both in vitro and in vivo methodologies, and the average absolute error was <1.2 degrees and <1.1 mm. RESULTS: At 90 degrees of knee flexion, the patella showed lateral spin (the distal pole of the patella rotated laterally) in subjects with patellofemoral pain (mean and standard deviation, -10.13 degrees +/- 2.24 degrees) and medial spin in healthy subjects (mean, 4.71 degrees +/- 1.17 degrees) (p < 0.001). At 90 degrees of knee flexion, the patella demonstrated significantly more lateral translation in subjects with patellofemoral pain (mean, 5.05 +/- 3.73 mm) than in healthy subjects (mean, -4.93 +/- 3.93 mm) (p < 0.001). CONCLUSIONS: Kinematic differences between healthy subjects and subjects with patellofemoral pain were demonstrated through a large, dynamic range of knee flexion angles. Increased lateral patellar translation and lateral patellar spin in subjects with patellofemoral pain suggest that the patella is not adequately balanced during functional activities in this group. Prospective studies are needed to identify when patellofemoral pain-related changes begin to occur and to determine the risk for the development of patellofemoral pain in individuals with abnormal kinematics.

ACL impingement prediction based on MRI scans of individual knees.

Although tibial external rotation and abduction do not load the ACL strongly in cadaver-based biomechanical studies, such knee positions are associated with ACL injuries in clinical practice. We hypothesized the ACL could be injured in such knee positions because of its impingement against the intercondylar notch. We developed a three-dimensional geometric ACL impingement model through segmentation of MR images of individual knees. We investigated impingement by determining the deformed geometry and elongation of the ligament as it wrapped around the notch surface during impingement. When impingement did not occur, the gap width separating the ligament and the notch surface was computed. Tibial external rotation/abduction could cause the ACL to impinge against the lateral notch wall and elongate as it wraps around the surface of the notch wall. The impingement occurred between the middle portion of the ligament (at 45% +/- 8% [mean +/- standard deviation] of the ligament length) and the convex surface of the lateral notch wall. Considering the multiband fiber architecture of the ligament, the anteromedial band of the ACL sustained greater elongation during impingement and showed a smaller gap width from the notch surface than the intermediate band or the posterolateral band.

Effect of different sitting postures on lung capacity, expiratory flow, and lumbar lordosis.

OBJECTIVE: To investigate the effect of sitting posture on lung capacity and expiratory flow (LC-EF) and lumbar lordosis. DESIGN: Repeated measures on 1 group of subjects in 4 postures. SETTING: Laboratory. PARTICIPANTS: Seventy able-bodied volunteers. INTERVENTIONS: Postures were assumed randomly: normal, with full ischial support and flat lumbar support; slumped, with the pelvis positioned in the middle of seat while leaning against the backrest; against the back part of the seat without ischial support (WO-BPS), with partially removed ischial support and an enhanced lumbar support; and standing. MAIN OUTCOME MEASURES: For LC-EF, forced vital capacity, maximum forced expiratory flow, forced expiratory volume in 1 second, and peak expiratory flow; and lumbar lordosis. RESULTS: All LC-EF measures in standing were significantly superior to those in slumped and normal sitting, and 4 measures were significantly higher than in WO-BPS. In slumped sitting, LC-EF significantly decreased from that in normal sitting. WO-BPS sitting significantly increased 4 of the LC-EF measures from those in the normal sitting. Lumbar lordosis was the highest in standing and progressively decreased in WO-BPS, normal, and slumped sitting. CONCLUSIONS: Slumped sitting significantly decreased LC-EF and lumbar lordosis. Because it increases the lumbar lordosis and promotes LC-EF, the WO-BPS posture may be a better seating option for people sitting for a prolonged time.

Sitting with adjustable ischial and back supports: biomechanical changes.

STUDY DESIGN: The seat and back contact force, pressure distribution, lumbar lordosis, and low back muscle activities associated with a new seat design with adjustable ischial support and backrest were investigated using kinematic, kinetic, electromyographic, and radiographic measurements. OBJECTIVES: To investigate the biomechanical effects of adjusting ischial and backrest supports during sitting. SUMMARY OF THE BACKGROUND DATA: Sitting may induce posterior rotation of the pelvis, reduction of lumbar lordosis, and increases in muscle tension, disc pressure, and pressure on the ischium and coccyx, which may be associated with low back pain. A device that reduces the ischial load and maintains lumbar lordosis may help increase seating comfort and reduce low back pain. METHODS: Fifteen office workers with no known low back pain history were tested. Contact pressure distributions, reaction forces between the buttock-thighs and seat and between the back and backrest, load carried by the seat pan and backrest, sacral inclination, lumbar lordosis, intervertebral space of lumbar spine, and muscular activity in stabilizing the trunk were measured for sitting with and without ischial support and with adjustable back support. RESULTS: When the ischial support was relieved, the center of the force on the seat and on the legs of the chair, and the peak center of pressure on the seat, were significantly (P < 0.002) shifted forward toward the thighs. The total contact area on the seat pan and on the backrest was significantly decreased and increased, respectively (P < 0.001). The sacral inclination, total and segmental lumbar lordosis, and lumbar spine disc height were significantly increased for sitting upright with backrest, with the lumbar curve close to that during standing. CONCLUSIONS: Sitting with reduced ischial support and fitted backrest to the lower spine altered the contact area, reduced peak pressure under the ischia, reduced muscular activity, maintained total and segmental lumbar lordosis, rotated the sacrum forward, and increased lumbar intervertebral disc heights, which could potentially reduce low back pain.

Preiser's disease: identification of two patterns.

PURPOSE: A large series of patients with Preiser's disease was reviewed to compare 2 potentially different categories of this disorder: complete versus partial vascular impairment of the scaphoid bone as determined by magnetic resonance imaging (MRI). METHODS: Nineteen patients with Preiser's disease were identified retrospectively from 2 institutions. Using MRI criteria, 2 disease patterns were identified: diffuse necrosis and/or ischemia of the scaphoid (type 1 disease, 11 cases) and segmental vascular impairment of the scaphoid (type 2 disease, 8 cases). Risk factors for osteonecrosis, treatment methods, and serial radiographs were reviewed in all cases. Sixteen patients were examined for the purpose of this study at an average follow-up of 25 months. RESULTS: MRI signal changes of necrosis and/or ischemia involved 100% of the scaphoid in type 1 cases and on average approximately 42% in type 2 cases (range, 33% to 66%). In type 1 cases, regardless of the treatment used, the scaphoid typically fragmented and collapsed. In type 2 cases, scaphoid architecture was altered minimally after similar treatment methods. A history of wrist trauma was significantly more common in type 2 cases, and the results of treatment were generally better in this group of patients (mean Mayo modified wrist scores, 86 vs 58 points). CONCLUSIONS: This study supports the concept of 2 patterns of scaphoid involvement in Preiser's disease. Type 1 cases are characterized by MRI signal changes of necrosis and/or ischemia involving the entire scaphoid bone. Patients in this group have a propensity for scaphoid deterioration. Type 2 cases have MRI signal changes involving only part of the scaphoid. These patients commonly report a history of wrist trauma, show fewer tendencies toward scaphoid fragmentation, and may have a more favorable clinical outcome.