Stress fractures occurring in the pubis and ischium
after peri-acetabular osteotomy (PAO) are not well recognised, with
a reported incidence of 2% to 3%. The purpose of this study was
to analyse the incidence of stress fracture after Bernese PAO under
the care of two high-volume surgeons. The study included 359 patients
(48 men, 311 women) operated on at a mean age of 31.1 years (15
to 56), with a mean follow-up of 26 months (6 to 64). Complete follow-up
radiographs were available for 348 patients, 64 of whom (18.4%)
developed a stress fracture of the inferior pubic ramus, which was
noted at a mean of 9.1 weeks (5 to 55) after surgery. Most (58;
91%) healed. In 40 of the patients with a stress fracture (62.5%),
pubic nonunion also occurred. Those with a stress fracture were
significantly older (mean 33.9 years (16 to 50) Cite this article:
The position of the pelvis has been shown to influence acetabular orientation. However there have been no studies quantifying that effect on the native acetabulum. Our aims were to investigate whether it is possible to quantify the relationship between pelvic tilt and acetabular orientation in native hips, and whether pelvic tilt affects acetabular cover of the femoral head. Computerized tomography scans of 93 hips (36 normal, 31 dysplastic and 26 with acetabular retroversion) were analyzed. We used a CT technique that allows standardised three-dimensional (3D) analysis of acetabular inclination and anteversion and calculation of femoral head cover in relation to the anterior pelvic plane and at different degrees of forward and backward tilt. Acetabular anteversion, inclination and cover of the femoral head were measured at pelvic tilt angles ranging from −20° to 20° in relation to the anterior pelvic plane using 5° increments. The effect of pelvic tilt on version was similar in the normal, dysplastic and retroverted groups, with a drop in anteversion ranging from 2.5° to 5° for every 5° of forward tilt. The effect on inclination was less marked and varied among the three groups. Pelvic tilt increased femoral head cover in both normal and dysplastic hips. The effect was less marked, and tended to be negligible at higher positive tilt angles, in the retroverted group. This study has provided benchmark data on how pelvic tilt affects various acetabular parameters which in turn may be helpful in promoting greater understanding of acetabular abnormalities and how pelvic tilt affects the interpretation of pelvic radiographs.
The position of the pelvis influences acetabular orientation. In particular, pelvic tilt in the sagittal plane may lead to inaccurate interpretation of plain pelvic radiographs. We therefore quantified the relationship between this pelvic tilt and acetabular orientation in native hips, and determined whether pelvic tilt affects femoral head cover. We analysed computed tomography scans of 93 hips (36 normal, 31 dysplastic, 26 with acetabular retroversion) and measured acetabular anteversion, inclination, and femoral head cover at pelvic tilt angles ranging from −20° to 20° in relation to the anterior pelvic plane using 5° increments. The effect of pelvic tilt on version was similar in the normal, dysplastic, and retroverted groups, with a drop in anteversion ranging from 2.5° to 5° for every 5° of forward tilt. There was a tendency for the inclination angle to decrease when the pelvis was tilted forward from a position of extension, and in normal hips, this produced a reduction in inclination of about 4° for every 8° of pelvic tilt; but once neutral pelvic tilt was reached, further forward rotation of the acetabulum had rather a small effect on the inclination angle. In normal and dysplastic hips pelvic tilt increased apparent femoral head cover; in the retroverted group the effect was less marked and tended to be negligible at higher tilt angles. Anterior cover increased with increasing forward tilt in all three groups of hips. Posterior cover, on the other hand, decreased by just 2% for the dysplastic hips, 3.5% for the normal hips, and 6% for the retroverted hips over the whole range of tilt from −20° to 20°. A greater understanding of the influence of pelvic tilt may allow improvements in the radiological diagnosis and surgical treatment of acetabular abnormalities, particularly in relation to acetabular reorientation procedures and femoroacetabular impingement.
In the sagital view, the centre of the circle is offset by 21mm (S.D.3mm) at an angle of 67° (S.D. 7°) from a line connecting the midpoint between the centres of the femoral condyles and the femoral head centre. On either end of this line, the articular surface of the trochlea can be fitted to spheres of radius 30mm (S.D. 6mm) laterally and 27mm (S.D. 5mm) medially, with an rms of 0.4mm.
This can be of use in planning and performing joint reconstruction and have implications for the design of patello-femoral replacements and the rules governing their position.
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.
Radiological measurements are an essential component of the assessment of outcome following knee arthroplasty. However, plain radiographic techniques can be associated with significant projectional errors because they are a two-dimensional (2D) representation of a three-dimensional (3D) structure. Angles that are considered within the target zone on one film may be outside that zone on other films. Moreover, these parameters can be subject to significant inter-observer differences when measured. The aim of our study therefore was to quantify the variability between observers evaluating plain radiographs following Unicompartmental knee arthroplasty. Twenty-three observers, made up of Orthopaedic Consultants and trainees, were asked to measure the coronal and sagittal alignment of the tibial and femoral components from the post-operative long-leg plain radiograph of a Unicompartmental knee arthroplasty. A post-operative CT scan using the low dose Imperial knee protocol was obtained as well and analysed with 3D reconstruction software to measure the true values of these parameters. The accuracy and spread of the pain radiographic measurements were then compared with the values obtained on the CT. On the femoral side, the mean angle in coronal alignment was 1.5° varus (Range 3.8, SD 1, min 0.1, max 3.9), whereas the mean angle in sagittal alignment was 8.6° of flexion (Range 7.5, SD 1.5, Min 3.7, Max 11.2). The true values measured with CT were 2.4° and 11.0° respectively. As for the tibial component, the mean coronal alignment angle was 89.7° (Range 11.6, SD 3.3, Min 83.8, Max 95.4), and the mean posterior slope was 2.4° (Range 8.7, SD 1.6, Min -2, Max 6.7). The CT values for these were 87.6° and 2.7° respectively. We conclude that the plain radiographic measurements had a large scatter evidenced by the wide ranges in the values obtained by the different observers. If only the means are compared, the plain radiographic values were comparable with the true values obtained with CT (that is; accuracy was good) with differences ranging from 0.3° to 2.4°. The lack of precision can be avoided with the use of CT, particularly with the advent of low-dose scanning protocols.
This study examined the relationship between the cross-over sign and the true three-dimensional anatomical version of the acetabulum. We also investigated whether in true retroversion there is excessive femoral head cover anteriorly. Radiographs of 64 hips in patients being investigated for symptoms of femoro-acetabular impingement were analysed and the presence of a cross-over sign was documented. CT scans of the same hips were analysed to determine anatomical version and femoral head cover in relation to the anterior pelvic plane after correcting for pelvic tilt. The sensitivity and specificity of the cross-over sign were 92% and 55%, respectively for identifying true acetabular retroversion. There was no significant difference in total cover between normal and retroverted cases. Anterior and posterior cover were, however, significantly different (p <
0.001 and 0.002). The cross-over sign was found to be sensitive but not specific. The results for femoral head cover suggest that retroversion is characterised by posterior deficiency but increased cover anteriorly.
We present a new CT-based method which measures cover of the femoral head in both normal and dysplastic hips and allows assessment of acetabular inclination and anteversion. A clear topographical image of the head with its covered area is generated. We studied 36 normal and 39 dysplastic hips. In the normal hips the mean cover was 73% (66% to 81%), whereas in the dysplastic group it was 51% (38% to 64%). The significant advantage of this technique is that it allows the measurements to be standardised with reference to a specific anatomical plane. When this is applied to assessing cover in surgery for dysplasia of the hip it gives a clearer understanding of where the corrected hip stands in relation to normal and allows accurate assessment of inclination and anteversion.
The rotational alignment of the tibia is an unresolved issue in knee replacement. A poor functional outcome may be due to malrotation of the tibial component. Our aim was to find a reliable method for positioning the tibial component in knee replacement. CT scans of 19 knees were reconstructed in three dimensions and orientated vertically. An axial plane was identified 20 mm below the tibial spines. The centre of each tibial condyle was calculated from ten points taken round the condylar cortex. The tibial tubercle centre was also generated as the centre of the circle which best fitted eight points on the outside of the tubercle in an axial plane at the level of its most prominent point. The derived points were identified by three observers with errors of 0.6 mm to 1 mm. The medial and lateral tibial centres were constant features (radius 24 mm ( Alignment of the knee when based on this anatomical axis was more reliable than either the posterior surfaces or any axis involving the tubercle which was the least reliable landmark in the region.
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.
