Acetabular centre positioning in the pelvis has a profound effect on hip joint function. The force–and moment-generating capacities of the hip muscles are highly sensitive to the location of the hip centre. We describe a novel 3D CT-based system that provides a scaled frame of reference (FOR) defining the hip centre coordinates in relation to easily identifiable pelvic anatomic landmarks. This FOR is more specific than the anterior pelvic plane (APP) alone, giving depth, height and width to the pelvis for both men and women under-going hip surgery. CT scans of 22 normal hips were analysed. There were 14 female and 8 male hips. The APP was used as the basis of the coordinate system with the origin set at the right anterior superior iliac spine. After aligning the pelvis with the APP, the pelvic horizontal dimension (Dx) was defined as the distance between the most lateral points on the iliac crests, and its vertical dimension (Dy) was the distance between the highest point on the iliac wing and the lowest point on ischial tuberosity. The pelvic depth (Dz) was defined as the horizontal distance between the posterior superior iliac spine and the ipsilateral ASIS. The ratios of the hip centre’s x, y, and z coordinates to their corresponding pelvic dimensions (Cx/Dx, Cy/Dy, Cz,Dz) were calculated. The results were analysed for men and women. For a given individual the hip centre coordinates can be derived from pelvic landmarks. We have found that the mean Cx/Dx measured 0.09 ± 0.02 (0.10 for males, 0.08 for females), Cy/Dy was 0.33 ± 0.02 (0.30 for males, 0.35 for females), and Cz/Dz was 0.37 ± 0.02 (0.39 for males and 0.36 for females). There was a statistically significant gender difference in Cy/Dy (p=0.0001) and Cz/Dz (p=0.03), but not in Cx/Dx (p=0.17). Anteversion for the male hips averaged 19° ± 3°, and for the female hips it was 26° ± 5°. Inclination measured 56° ± 1° for the males and 55° ± 4° for the females. Reliability testing showed a mean intra-class correlation coefficient of 0.95. Bland-Altman plots showed a good inter-observer agreement. This method relies on a small number of anatomical points that are easily identifiable. The fairly constant relationship between the centre coordinates and pelvic dimensions allows derivation of the hip centre position from those dimensions. Even in this small group, it is apparent that there is a difference between the sexes in all three dimensions. Without the need for detailed imaging, the pelvic points allow the surgeon to scale the patient’s pelvis and thereby know within a few millimetres the ‘normal’ position of the acetabulum for both men and women. This knowledge may be of benefit when planning or undertaking reconstructive hip surgery especially in patients with hip dysplasia or bilateral hip disease where there is no reference available for planning the surgery.
Although acetabular centre positioning has a profound effect on hip joint function, there are very few studies describing accurate methods of defining the acetabular centre position in 3D space. Clinical and plain radiographic methods are inaccurate and unreliable. We hypothesize that a 3D CT-based system would provide a gender-specific scaled frame of reference defining the hip centre coordinates in relation to easily identifiable pelvic anatomic landmarks. CT scans of thirty-seven normal hips (19 female and 18 male) were analysed. The ratios of the hip centre coordinates to their corresponding pelvic dimensions represented its horizontal (x), vertical (y), and posterior (z) scaled offsets (HSO, VSO, and PSO). The mean HSO for females was 0.08 ± 0.018, mean VSO was 0.35 ± 0.018, and mean PSO was 0.36 ± 0.017. For males HSO averaged 0.10 ± 0.014, VSO was 0.32 ± 0.015, and PSO was 0.38 ± 0.013. There was a statistically significant gender difference in all three scaled offsets (p=0.04, 0.002, and 0.03 for HSO, VSO, and PSO respectively). Inter-observer agreement tests showed a mean intra-class correlation coefficient of 0.95. We conclude that this frame of reference is gender-specific giving a unique scale to the patient and allowing reliable derivation of the position of the hip centre from the pelvic dimensions alone. The gender differences should be borne in mind when positioning the centre of a reconstructed hip joint. Using this method, malpositioning, particularly in the antero-posterior (or z) axis, can be identified and addressed in a malfunctioning hip replacement. Pathological states, such as dysplasia and protrusio, can also be accurately described and surgery addressing them can be precisely planned.
The rotational alignment of the tibia is an as yet unresolved issue for arthroplasty surgeons. Functional variation may be due to minor malrotation of the tibial component. The aim was to find a reliable method for positioning the tibial component in arthroplasty. CT scans of 21 knees were reconstructed in three dimensions and oriented vertically. A plane was taken 20 mm below the tibial spines. The centre of each tibial condyle was calculated from points taken round that condylar cortex. A tibial tubercle centre was also generated as the centre of the circle that best fit points on the surface of the tubercle in the plane of its most prominent point. The derived points were identified by three observers with errors of 0.6 – 1mm. The medial and lateral tibial centres were constant features (radius 24mm ± 3mm, and 22mm ± 3mm respectively). An ‘anatomic’ axis was created perpendicular to a line joining these two points. The tubercle centre was found 20mm ± 7mm lateral to the medial tibial centre. Compared to this axis, an axis perpendicular to the posterior condylar axis was internally rotated by 6° ± 3°. An axis based on the tibial tubercle and the tibial spines was also internally rotated by 6° ± 10°. We conclude that alignment of the knee when based on this ‘anatomic’ axis is more reliable than either of the posterior surfaces. It is also more reliable than any axis involving the tubercle, which is the least reliable feature in the region. The ‘anatomic’ axis can be used in navigated knee arthroplasty for referencing the rotational alignment of the tibial component.
Using software adapted for the specific purpose, femoral cortical volume was measured at three different levels at a fixed distance from the lower border of the lesser trochanter on both sides: 6mm distal to the tip of the prosthesis (z), At the top of the cylindrical portion(x) Midway between x and z (y). Accuracy and precision of the of the method was also assessed.
No significant trend was noted with change of volume of bone with time. In the three cases who had cemented hips on their other side, the cemented hips exhibited substantially more stress shielding than their cementless controls (ratios of 0.82, 0.74 and 0.85). A high correlation between the test and standard measurements was noted. The interobserver agreement between two observers was also good.
Conventional instrumentation, 3D plan based on a CT scan of the particular bone, helped by a conventional jig Navigation system. This achieved angle was then compared with the angle originally planned for each bone in all three groups using digitizing arm.
Wear and loosening are the major causes for long tem failure in Total Hip Replacement (THR). Accurate three dimensional wear analysis of radiographs has its own limitations. We report the results of our clinical study of three dimensional volumetric wear measurements using our custom low radiation risk CT based algorithm and special software Twenty four patients (32 hips) agreed to take part in our study. The male: female ratio was 1:4. The mean age was 75 years and the mean follow up was 5.4 years. All patients had 28 mm diameter ceramic heads. Of the 32 hips, 17 hips had polyethylene inserts and 15 hips had ceramic inserts. The maximum follow up for the polyethylene and ceramic groups were 12 years and 5.5 years respectively. All the patients were scanned using Somatom Sensation 4 scanner. Using custom software, 3D reconstruction of the components was done and landmark acquisition done on the femoral head, acetabular metal component and the insert. From these landmarks, a dedicated program was used to calculate the centre of the femoral head in relation to the centre of the acetabular component in all three axes and an indirect measurement of wear obtained. Using the axes measurements graphical 3D models of migration of the femoral head component into the acetabular liner were created and volume of wear measured using special software. Accuracy of the method was assessed by measuring the radius of the femoral head since all patients had 28mm diameter heads implanted in them. Assessment of precision of method was done by calculating the level of agreement between two independent observers. In the polyethylene group, there was no significant (<
1mm) wear in x and y axis with time. However there was significant evidence of wear in relation to time in the z axis (max wear = −2.5 mm). In the ceramic group with relatively shorter follow up, there was no evidence of significant wear in all three axes. The mean volume measured in the polyethylene group was 685 mm3 (max = 1629 mm3, min = 132mm3 ). The mean volume measured in the ceramic group was 350mm3 (max = 1045 mm3, min = 139mm3 ). The mean radius of the femoral head measured in both groups was 14.02mm (range =13.8 to 14.4 mm). Accuracy was limited by artifacts particularly in bilateral hip arthroplasties and further in the ceramic group because of the restricted access to the ceramic head for placement of markers. Measurements obtained by two independent observers showed a strong correlation (0.99, p value = 0.001) for the polyethylene group. In the ceramic group the correlation (0.69, p value=0.0126) was not as strong as the polyethylene group. This study has produced a method for three dimensional estimation of wear that can be obtained from low dose CT scans with better accuracy and repeatability (<
0.5 mm) even than to ex vivo studies particularly in polyethylene bearings(wear rate 0.14mm/yr). Noise reduction with appropriate artefact reduction software may further improve the accuracy of this simple and repeatable method.
Periprosthetic bone remodeling after uncemented hip replacement has always been a matter of research and debate. DEXA analysis of bone density was studied by previous groups but not the cross sectional cortical volume. We report a validated CT based algorithm for accurate measurement of cortical volume in these group of patients. Twenty two patients who have undergone Uncemented Furlong total hip replacement agreed to undergo CT scan of their hips for our study. The mean age was 74.6 yrs. The mean follow up was 5.4 yrs. Using software adapted for the specific purpose, femoral cortical volume was measured at three different levels at a fixed distance from the lower border of the lesser trochanter on both sides:
6mm distal to the tip of the prosthesis (z), At the top of the cylindrical portion(x) Midway between x and z (y). Accuracy of the method was assessed by measuring the volume of artificial cavities created on a polyurethane pelvis. Assessment of precision of method was done by calculating the level of agreement between two observers. The mean cortical volume in the proximal cylindrical portion (x), midpoint(y) and the portion of bone distal to the prosthesis (z) were 458 mm3, 466 mm3, 504 mm3 respectively. The corresponding cortical volumes in the contralateral native femur in unilateral hip replacements were 530 mm3(x), 511 mm3(y), 522 mm3 (z) giving a ratios of 0.86(x), 0.91(y) and 0.97(z). The mean cortical volumes on the left side of bilateral hips were 490 mm3(x), 499 mm3(y) and 528 mm3 (z). The mean cortical volumes on the right side were 456 mm3(x), 463 mm3 (y) and 516 mm3 (z). No significant trend was noted with change of volume of bone with time. In the three cases who had cemented hips on their other side, the cemented hips exhibited substantially more stress shielding than their cementless controls (ratios of 0.82, 0.74 and 0.85). A high correlation between the test and standard measurements was noted. The interobserver agreement between two observers was also good. In a fully coated uncemented femoral component, with documented long term results, it is to be expected that load will be shed steadily along the length of the prosthesis. In this study we have confirmed this supposition, with volumetric data, by showing that an almost normal bone just below the tip of the stem (97% volume) reduces to a bone volume of 91% by the middle of the stem and then 86% by the shoulder of the prosthesis. This decrease in the volume of cortical bone effectively normal at the tip of the prosthesis while not optimal appears to stabilize early with no trend of continued reduction over a decade. The effect of cementation on stress shielding was only examined incidentally in this study but appears to contribute to more marked bone loss.
Periacetabular osteolysis is now considered one of the major long term complications following uncemented total hip replacement. Radiographs are inaccurate and lack sensitivity in detecting lesions even with multiple views. Very few clinical studies have shown the use of CTscan for measuring these lesions. We report our clinical experience with CT based algorithm for measuring it. Twenty two patients (32 hips) who have undergone Uncemented Furlong total hip replacement agreed to undergo CT scan of their hips for our study. The mean follow up was 5.4 yrs. Of the 34 hips,17 were polyethylene bearings and 15 were ceramic bearings. Nine patients had bilateral replacement in this group. Using custom reconstruction software, 3D models were created and volume measurements made after identifying the lesions in the slices and painting them using appropriate tools available in the software. Accuracy of the method was assessed by measuring the volume of artificial cavities created on polyurethane pelvis with and without the components. In our control experiments, a high correlation between the test and standard measurements was noted in the cavities above the component, while medial to the acetabular component in bilateral cases it was difficult to be accurate, with cavities less than 10mm in diameter being hard to detect reliably. In our clinical group of 32 hips, degenerative cysts were noted in 13, secondary rheumatoid cysts in 2 and wear cysts were noted in 2, the largest having a maximum dimension of 10mm. All the degenerative cysts were in the peripheral zone and both the wear cysts were seen in the central zone communicating with the screw holes. These cysts were identified by the characteristic absence of sclerosis surrounding the cyst and obvious communication with screw holes. Both the wear cysts were found with polyethylene bearings at a minimum of 5yrs follow up. The mean volume of the degenerative cysts was 799 mm3 (71–3500) and the mean volume of the wear cysts was 567 mm3 (550–585) The low dose CT method we describe and the results we report show that cavities can be measured reliably, above or below the acetabular component. On the medial side, in bilateral cases in particular, although location is possible, volumetric analysis of anything less than 10mm in diameter is not. Regarding surveillance strategy for wear cysts, we have established that in this series the incidence is 14%, with one at 5 yrs and another noted at 12 yrs, with 10mm in maximum dimension. The absence of any wear cysts at all in the ceramic group, albeit after a shorter follow up of only 5 years is encouraging. Based on these figures, with these implants, we would recommend that there is no need to undertake surveillance more frequently than every 10 years.
Assessing femoral head coverage is a crucial element in acetabular surgery for hip dysplasia. Plain radiographic indices give rather limited information. We present a novel CT-based method that measures the fraction of the femoral head that is covered by the acetabulum. This method also produces a direct image of the femoral head with the covered part clearly represented, and it also measures acetabular inclination and anteversion. We used this method to determine normal coverage, and applied it to a prospective study of patients with hip dysplasia undergoing periacetabular osteotomy. Twenty-five normal and 26 dysplastic hips were studied. On each CT scan points were assigned on the femoral head surface and the superior half of the acetabular rim. The anterior pelvic plane was then defined, and the pelvis was aligned in that plane. Using our custom software programme, the fraction of the head that was covered was measured, in addition to acetabular inclination and anteversion. In the normal hips femoral head coverage averaged 73% (SD 4). In the same group, mean anteversion was 15.7° (SD 7°), whereas mean inclination was 44.4° (SD 4°). In the dysplastic group femoral head coverage averaged 50.3% (SD 6), whereas mean anteversion and inclination were 18.7° (SD 9°) and 53.2° (SD 5°) respectively. This is the first study to our knowledge that has used a reliable measurement technique of femoral head coverage by the acetabulum in the normal hip. When this is applied to assessing coverage in surgery for hip dysplasia it allows a clearer understanding of where the corrected hip stands in relation to a normal hip. This would then allow for better determination of the likely outcome of this type of surgery. We are presently conducting a prospective study using this technique to study dysplastic hips pre- and post-periacetabular osteotomy.
Whilst computer assistance enables more accurate arthroplasty to be performed, demonstrating this is difficult. The superior results of CAOS systems have not been widely appreciated because accurate determination of the position of the implants is impossible with conventional radiographs for they give very little information outside their plane of view. We report on the use of low dose (approximately a quarter of a conventional pelvic scan), low cost CT to robustly measure and demonstrate the efficacy of computer assisted hip resurfacing. In this study we demonstrate 3 methods of using 3D CT to measure the difference between the planned and achieved positions in both conventional and navigated hip resurfacing. The initial part of this study was performed by imaging a standard radiological, tissue equivalent phantom pelvis. The 3D surface models extracted from the CT scan were co-registered with a further scan of the same phantom. Subsequently both the femoral and acetabular components were scanned encased in a large block of ice to simulate the equivalent Hounsfield value of human tissue. The CT images of the metal components were then co-registered with their digital images provided by the implant manufactures. The accuracy of the co-registration algorithm developed here was shown to be within 0.5mm. This technique was subsequently used to evaluate the accuracy of component placement in our patients who were all pre-operatively CT scanned. Their surgery was digitally planned by first defining the anterior pelvic plane (APP), which is then used as the frame of reference to accurately position and size the wire frame models of the implant. This plan greatly aids the surgeon in both groups and in the computer assisted arm the Acrobot Wayfinder uses this pre-operative plan to guide the surgeon. Following surgery all patients, in both groups were further CT scanned to evaluate the achieved accuracy. This post-operative CT scan is co-registered to the pre-operative CT based plan. The difference between the planned and achieved implant positions is accurately computed in all three planes, giving 3 angular and 3 translational numerical values for each component. Further analysis of the CT generated results is used to measure the implant intersection volume between the pre-operatively planned and achieved positions. This gives a single numerical value of placement error for each component. These 3D CT datasets have also been used to quantify the volume of bone resected in both groups of patients comparing the simulated resection of the planned position of the implant to that measured on the post-operative CT. This study uses 3D CT as a surrogate outcome measure to demonstrate the efficacy of CAOS systems.
Complications following hip resurfacing occur primarily because of the surgeon’s inability to achieve optimal implant positioning, and the significant learning curve associated with it. Our study sought to look at the impact of navigation technologies on this learning curve. Twenty medical students doing their BSc project took part in the study. Four types of synthetic femurs were used for the study viz., Normal anatomy (11students), Osteoarthritis (5), Coxa Vara(2) and Coxa Valga(2). Each student was allowed to insert the guide wire according to their judgement in the femoral head using 3 systems: Conventional instrumentation, 3D plan based on a CT scan of the particular bone, helped by a conventional jig and Navigation system. This achieved angle was then compared with the angle originally planned for each bone in all three groups using digitizing arm. The range of error using the conventional method to insert a guide wire was 23deg (range −9 to 14, SD= 6.3), using the CT plan method, it was 22 deg (range −9 to 13, SD=6.6). Using the Navigation method it was 7 deg (range −5 to 2, SD=2.). Students who progressed from conventional through planning to navigation (group 1) were no more accurate than students who went straight to navigation without ever having used conventional instrumentation (group 3). Students produced similar accuracy even in their maiden attempt, on difficult anatomy when provided with navigation technology. This study has shown that motivated and enthusiastic students can achieve an expert level of accuracy very rapidly when provided with the appropriate level of technology. he development of surgeons who are able to deliver excellent outcomes depends more on technology than training.
Last year at CAOS UK we reported on the development of the Acrobot® Navigation System for accurate computer-assisted hip resurfacing surgery. This paper describes the findings of using the system in the clinical setting and includes the improvements that have been made to expedite the procedure. The aim of our system is to allow accurate planning of the surgery and precise placement of the prosthesis in accordance with the plan, with a zero intra-operative time penalty in comparison to the standard non-navigated technique. The system uses a pre-operative CT-based plan to allow the surgeon to have full 3D knowledge of the patient’s anatomy and complete control over the sizes and positions of the components prior to surgery. At present the navigation system is undergoing final clinical evaluation prior to a clinical study designed to demonstrate the accuracy of outcome compared with the conventional technique. Whilst full results are not yet available, this paper describes the techniques that are being used to evaluate accuracy by comparing pre-operative CT-based plans with post-operative CT scans, and gives initial results. This approach provides a true measure of procedure outcome by measuring what was achieved against what was planned in 3D. The measure includes all the sources of error present within the procedure protocol, therefore these results represent the first time that the outcome of a navigated orthopaedic procedure has been measured accurately.
With regard to functional activity, 21% of patients in both the groups scored 8 or more on the UCLA activity scale. 21% of patients in the BHR and 14% in the Furlong group scored 3 or more on the Tegner activity scale
