The aim of this retrospective cohort study was to identify any difference in femoral offset as measured on pre-operative anteroposterior (AP) radiographs of the pelvis, AP radiographs of the hip and corresponding CT scans in a consecutive series of 100 patients with primary end-stage osteoarthritis of the hip (43 men and 57 women with a mean age of 61 years (45 to 74) and a mean body mass index of 28 kg/m2 (20 to 45)).
Patients were positioned according to a standardised protocol to achieve reproducible projection and all images were calibrated. Inter- and intra-observer reliability was evaluated and agreement between methods was assessed using Bland-Altman plots.
In the entire cohort, the mean femoral offset was 39.0 mm (95% confidence interval (CI) 37.4 to 40.6) on radiographs of the pelvis, 44.0 mm (95% CI 42.4 to 45.6) on radiographs of the hip and 44.7 mm (95% CI 43.5 to 45.9) on CT scans. AP radiographs of the pelvis underestimated femoral offset by 13% when compared with CT (p < 0.001). No difference in mean femoral offset was seen between AP radiographs of the hip and CT (p = 0.191).
Our results suggest that femoral offset is significantly underestimated on AP radiographs of the pelvis but can be reliably and accurately assessed on AP radiographs of the hip in patients with primary end-stage hip osteoarthritis.
We, therefore, recommend that additional AP radiographs of the hip are obtained routinely for the pre-operative assessment of femoral offset when templating before total hip replacement.
The accurate restoration of physiological joint biomechanics by total hip replacement (THR) results in improved abductor muscle strength1,2 and a greater range of movement (ROM).1,3-5 It also reduces the risk of post-operative complications such as limp, dislocation and wear-related implant failure.3,6-8
For precise pre-operative planning, the accurate assessment of femoral offset (FO) is essential, helping to restore a physiological biomechanical environment. In clinical practice, pre-operative templating is predominantly performed on anteroposterior (AP) radiographs of the pelvis, even though several studies over the past two decades have shown that the measurement of femoral offset on AP radiographs of the pelvis is not as reliable as the use of CT.9-12 Femoral offset tends to be underestimated on AP radiographs of the pelvis because of the projectional effects of femoral anteversion (FA)10,11 and external rotational contracture in patients with end-stage osteoarthritis (OA) of the hip.9,12 Consequently, some authors have proposed pre-operative CT as a standard procedure.10,11 Although CT is considered to be the best method for assessing femoral offset,13,14 its routine usage has to be questioned because of its higher radiation dose, higher cost and limited availability.
To the best of our knowledge, no previous studies have investigated the relative accuracy and reliability of measurements of femoral offset on AP radiographs of the hip and CT in the pre-operative planning of a THR. The aim of the present study was to evaluate differences in offset when measured on standardised AP radiographs of the pelvis, AP radiographs of the hip and CT scans of patients with primary OA of the hip. Our principle hypothesis was that AP radiographs of the pelvis significantly underestimate femoral offset compared with AP radiographs of the hip. Secondly, we hypothesised that there is no clinically relevant difference between the mean measurements of femoral offset from AP radiographs of the hip and CT scans.
Patients and Methods
We retrospectively reviewed a consecutive series of 152 patients who had undergone a primary THR with a custom-made cementless femoral component15 for primary OA of the hip between July 2009 and December 2009. For each patient, a pre-operative AP radiograph of the pelvis and an AP radiograph and CT scan of the affected hip had been obtained. All images were retrieved in DICOM format from a picture archiving and communication system (PACS).
We excluded patients with secondary OA of the hip, those on medication which affected bone metabolism and those whose radiographs did not have a calibration marker. Patients who underwent bilateral THR during this period only had the first procedure included in the study. In all, 52 patients were excluded, leaving 100 patients in the study (43 men and 57 women; mean age 61 years (45 to 74); mean body mass index (BMI) 28 kg/m2 (20 to 45)). The study was approved by the institutional review board.
In each case, a standardised protocol was used to achieve reproducible projection. Low-centred AP radiographs of the pelvis and hip were taken in the supine position (Fig. 1). In order to correct for the effects of magnification, a metal calibration sphere of 25 mm was placed on the inner thigh at the level of the femoral head in AP projection. During the study period, two different x-ray tubes were in use: a Canon CXDI series (Canon Inc., Tokyo, Japan) and a Philips Bucky Diagnost VE VT (Royal Philips Electronics Inc., Amsterdam, Netherlands). The tube-to-film distance was 1150 mm, with the tube orientated perpendicular to the table.
For AP radiographs of the pelvis, both legs were internally rotated by 15° using a leg retainer and the crosshair of the beam was centred on the symphysis pubis.
For AP radiographs of the affected hip, the crosshair of the beam was sited over the midpoint of a line between the anterior superior iliac spine and the symphysis pubis to position it over the centre of the femoral head. The affected leg was internally rotated and retained so that the prominence of the greater trochanter could be palpated in its most lateral position thereby aligning the femoral neck to the coronal plane. When the leg could not be internally rotated sufficiently because of an external rotation contracture, the whole affected hip was additionally elevated with a wedge placed under the buttock.
All CT scans were performed pre-operatively using a Toshiba Aquilion 16 CT scanner (Toshiba Corp., Tokyo, Japan). Each patient was positioned supine with their legs in neutral rotation as confirmed by scout views. The scans were obtained in three sets: the first from the cranial aspect of the acetabulum to below the lesser trochanter, the second from below the lesser trochanter to a point 50 mm distal to the femoral isthmus and the third, which consisted of four to six slices of the knee. The slices were spaced at 4 mm, 8 mm and 2 mm, respectively. Each scan was recorded with gantry tilt 0°, 120 kV and a field of view (FOV) of 250 mm.
A validated MATLAB program (version 7.10; The MathWorks Inc., Natick, Massachusetts) was used to determine the centre (HC) and diameter of the femoral head (HD) and the axis of the femoral shaft on AP projections of the pelvis and hip. Two points were defined on the medial and lateral cortex of the femoral diaphysis 20 mm below the lesser trochanter, and a further two points on the medial and lateral cortex at the level of the femoral isthmus. The midpoints of these pairs determined the axis of the femoral shaft. A circle tool was used to define the diameter of the head and the co-ordinates of its centre. The femoral offset (FO) was calculated as the perpendicular distance from the centre of the femoral head to the axis of the femoral shaft. These measurements were labelled FOp and HDp on AP radiographs of the pelvis, and FOh and HDh on AP radiographs of the hip (Fig. 1). We also calculated the distance from the midpoint of both tear drops to the centre of the femoral head (x) on the AP radiographs of the pelvis.
For three-dimensional (3D) measurements, a further validated MATLAB programme was used to measure FO and FA. This programme enabled the user to select points from pre-selected axial CT slices, and performed calculations in the 3D co-ordinate system of the CT scanner (Fig. 2).
For the 3D calculation of offset (FOc) and head diameter (HDc), three axial slices were selected (s1, s3, s4; Fig. 3). HDc and the centre of the femoral head were determined on the slice with the femoral head at its largest diameter (s1) using a circle tool. The axis of the femoral shaft was defined by the centroid of the proximal femoral metaphysis (s3)16,17 and the centre of the isthmus (s4); FOc was then calculated as the perpendicular distance from the axis of the femoral shaft to the centre of the femoral head.
For the calculation of FA, the axis of the femoral neck (s2, Fig. 3) was defined using the single slice method as described by Sugano,18 and the posterior axis of the condyles (s5) by the most posterior aspects of the lateral and medial condyles. The angle between the axis of the femoral neck and the axis of the posterior condyles represented the FA.
Intra- and interobserver reliabilities for 20 randomly-selected AP radiographs of the pelvis, AP radiographs of the hip and CT scans were evaluated by two independent blinded observers using single-measure intra-class-correlation coefficients (ICC) with a two-way random effects model for absolute agreement.
The distributions of variables were examined in exploratory data analysis, and tested for normality using Kolmogorov-Smirnov tests. All variables (FOp, FOh, FOc, HDp, HDh, HDc and FA) were normally distributed (p = 0.06 to 0.20). For descriptive analysis, absolute mean values and differences of FO were expressed in millimetres with 95% confidence intervals (CIs). FA was expressed in degrees with 95% CI. Distributions of FO values were compared using paired-samples t-tests for paired observations and independent-samples t-tests for unpaired observations. Pearson’s correlation coefficient (r) was used to evaluate associations between continuous variables. Correlation was characterised as poor (0.00 to 0.20), fair (0.21 to 0.40), moderate (0.41 to 0.60) good (0.61 to 0.80), or excellent (0.81 to 1.00). Results with p-values < 0.05 were considered to be significant: p-values of < 0.001 were considered to be highly significant. Differences in FO measurements on all three modes of radiological imaging were analysed using Bland-Altman plots.19 Linear regression was performed to relate corresponding measurements of FO. Statistical analysis was carried out using PASW Statistics 18 (SPSS Inc., Chicago, Illinois) and Sigmaplot 12.0 (Systat Inc., San Jose, California).
Excellent intra-observer ICC was seen for all measurements: FOp (0.993), FOh (0.990), FOc (0.990), HDp (0.886), HDh (0.906), HDc (0.933) and FA (0.984).
Interobserver ICC also showed similar correlations for all measurements: FOp (0.986), FOh (0.977), FOc (0.991), HDp (0.812), HDh (0.848), HDc (0.866) and FA (0.950).
The mean FOp of the whole cohort was 39.0 mm (95% CI 37.4 to 40.6) and the mean FOh was 44.0 mm (95% CI 42.4 to 45.6). The mean FOp was significantly lower than the mean FOh (p < 0.001, paired samples t-test) with a mean difference of 5.0 mm (95% CI 4.3 to 5.7) or 11% (Fig. 4). The difference between FOp and FOh was slightly higher in men (5.8 mm, 14%) than in women (4.4 mm, 12%; p = 0.45, independent samples t-test).
We found no correlation between the difference in mean FO on corresponding radiographs and FA (r = 0.18, p = 0.07). The observed mean difference between HDp and HDh (1.0 mm (95% CI -0.6 to 1.4)) was not statistically significant (p = 0.068, paired samples t-test). There was also excellent correlation between HDp and HDh (r = 0.89, p < 0.001).
The mean FOc was 44.7 mm (95% CI 43.5 to 45.9). FOp was underestimated by 5.7 mm (95% CI 4.5 to 6.9) or 13% with reference to FOc (p < 0.001, paired samples t-test). FOc and FOh were similar (p = 0.191, paired samples t-test) with a mean difference of 0.7 mm (95% CI -0.3 to 1.7) (Fig. 4). The differences between FOh and FOc were similar in both men and women (p = 0.79, independent samples t-test).
We noted a good correlation between FOp and FOc (r = 0.687, p < 0.001) and between FOh and FOc (r = 0.767, p < 0.001) (Fig. 5). When compared with the CT measurements, disagreement in assessment of FO was within ± 5 mm in 44% (44 patients) of cases for AP radiographs of the pelvis and 74% (74 patients) of cases for AP radiographs of the hip (Fig. 6).
Figs. 6a - 6b
The mean diameter of the femoral head as measured by CT (HDc) was less than the mean HDp (mean difference -1.2 mm (95% CI -2.0 to -0.4), p = 0.003, paired samples t-test) and HDh (mean difference -2.2 mm (95% CI -3.0 to -1.4), p < 0.001, paired samples t-test). Correlations between HDc and HDp (r = 0.62, p < 0.001) and HDc and HDp (p = 0.63, p < 0.001) were similar.
Accurate and reliable assessment of femoral offset is a key element in the pre-operative planning of a THR as it determines the selection and positioning of the prosthetic components. Restoration of offset is essential to achieve a stable articulation,20 a good range of movement,1 good muscle function1,2 and equal limb length.3 The importance of the restoration of offset in preventing long-term adverse effects related to impingement21 and wear8,22 is well accepted.
Despite the lesser accuracy of plain radiography when compared with 3D techniques based on CT,10 AP radiographs of the pelvis are widely used for pre-operative templating in primary hip OA as they provide essential information about the anatomy of the pelvis and contralateral hip and allow evaluation of leg-length discrepancy. It is well established that femoral rotation influences the radiological appearance of the proximal femur,23 and that failing to correct for femoral anteversion and external rotation contracture will result in underestimation of femoral offset.10,24 Consequently, clear recommendations have been made about how to position the femoral neck in the coronal plane by internally rotating the lower limb.25 Little attention has been given, however, to potential differences in the accuracy of assessment of offset on comparable AP radiographs of the pelvis and hip. In contrast to AP views of the pelvis, AP radiographs of the hip theoretically allow full correction of individual femoral version, even in the presence of an external rotation contracture, as the hip can be elevated and symmetry is not an issue. Moreover, the centre of the beam is directed to the centre of rotation of the diseased hip, which minimises the effects of radiographic beam divergence.
The present study evaluated differences in measurements of femoral offset performed on standardised AP radiographs of the pelvis and hip in patients with primary end-stage hip OA and compared these measurements to 3D offset values obtained from corresponding CT scans. We observed a significant underestimation of offset on AP radiographs of the pelvis with a mean difference of 5.7 mm (13%) when compared with CT based measurements. The measurements of offset performed on AP radiographs of the hip were comparable to CT-based measurements with a mean difference of 0.7 mm (2%), suggesting that measurements of offset performed on standardised AP views of the hip are more accurate and allow a better representation of 3D offset.
We acknowledge the following limitations of this study: first, we cannot, in retrospect, separately quantify the effects of positioning and centring of the beam. Although a standardised positioning protocol was used, the method for obtaining the radiographs remains a potential source of bias as both femoral rotation and positioning of the calibration marker depend on the judgment of the technician. Similarly, internal rotation of a retroverted femur during positioning of the patient may have exaggerated measurement errors of offset. There were, however, only seven retroverted femora in our cohort with a mean retroversion of -2.7° (95% CI -4.5 to -0.9)).
Secondly, the diameter of the femoral head as determined by CT was less than on the corresponding radiographs. It has been shown that the femoral head is not a perfect sphere26; consequently, the demonstrated differences may be explained by the fact that HD was measured in the transverse plane on CT, but in the frontal plane on radiographs. Moreover, the slice spacing of the CT protocol (4 mm) aimed to reduce radiation exposure and to minimise artefacts caused by contralateral implants. This may have impeded the selection of the true level of the largest head diameter in some cases, potentially contributing to the lower HD values obtained from CT.
Thirdly, in order to evaluate 3D FA, the true axis of the femoral neck can only be determined by means of a 3D reconstruction of the neck. This was a limitation of our CT protocol as a 1 mm slice interval is needed for this purpose. However, the selected single slice method chosen in this study has been shown to be sufficiently accurate to measure FA when the slice for determining the femoral neck axis is chosen from a point just below the femoral head.18 Moreover, we excluded patients with head or neck deformity associated with secondary OA.
Lastly, we did not use the long axis of the femur (centroid of metaphysis to centre of knee) as previously reported for measurements of femoral offset on CT.27 In the present study, measurements of offset were based on the longitudinal axis of the proximal femur (centroid of metaphysis to centre of isthmus) in order to replicate the measurements of offset made on the plain radiographs.
Because the accuracy of conventional radiographs of the pelvis in measuring femoral offset is limited, it has been suggested that CT scans are used to measure it pre-operatively.10,11 As a result of this study we would question this premise for patients with primary OA, as we did not see a clinically relevant difference in offset values between AP radiographs of the hip and those made on CT. The mean values which we obtained for femoral offset (44.0 mm) on AP views of the hip and offset (44.7 mm) and FA (14.9°) on CT scans are comparable to those reported in the literature.10,28,29 Moreover, the present study suggests that the 3D measurement of offset may potentially be predicted from measurements performed on AP views of the pelvis as a good correlation between corresponding measurements of offset was seen. However, a larger series of patients would be needed to confirm this finding and to obtain more powerful prediction equations for men and women separately.
A decrease in femoral offset of approximately 12% following THR has been reported to cause abductor weakness2 which suggests that the observed underestimation of offset between AP views of the pelvis and hip is of clinical relevance. The difference between the offset values on AP views of the hip and CT is likely to be of minor clinical importance. The observed differences in measurements were consistent when considering men and women separately, which suggests that our findings are independent of gender.30,31
In conclusion, this study shows that, when compared with CT, femoral offset is significantly underestimated on AP radiographs of the pelvis, but can be reliably measured with a higher level of accuracy on AP radiographs of the hip in patients with primary OA. AP radiographs of the hip allow better control of femoral rotation, even in the presence of an external rotation contracture, and minimise the adverse effects of radiological projection. Instead of the suggested routine performance of CT for the pre-operative assessment of offset, AP radiographs of the hip reduce the patient's exposure to radiation, are more cost-effective and are easily available. Although the ultimate choice of implant design, size and position must still be made intra-operatively, we recommend that an additional AP radiograph of the hip is obtained routinely for the pre-operative assessment of femoral offset in patients with primary OA.
1 McGrory BJ , MorreyBF, CahalanTD, AnKN, CabanelaME. Effect of femoral offset on range of motion and abductor muscle strength after total hip arthroplasty. J Bone Joint Surg [Br]1995;77-B:865–869. Google Scholar
2 Asayama I , ChamnongkichS, SimpsonKJ, KinseyTL, MahoneyOM. Reconstructed hip joint position and abductor muscle strength after total hip arthroplasty. J Arthroplasty2005;20:414–420. Google Scholar
3 Charles MN , BourneRB, DaveyJR, et al.Soft-tissue balancing of the hip: the role of femoral offset restoration. Instr Course Lect2005;54:131–141. Google Scholar
4 Matsushita A , NakashimaY, JingushiS, et al.Effects of the femoral offset and the head size on the safe range of motion in total hip arthroplasty. J Arthroplasty2009;24:646–651. Google Scholar
5 Sakai T , SuganoN, OhzonoK, et al.Femoral anteversion, femoral offset, and abductor lever arm after total hip arthroplasty using a modular femoral neck system. J Orthop Sci2002;7:62–67. Google Scholar
6 Lecerf G , FessyMH, PhilippotR, et al.Femoral offset: anatomical concept, definition, assessment, implications for preoperative templating and hip arthroplasty. Orthop Traumatol Surg Res2009;95:210–219. Google Scholar
7 Patel AB , WagleRR, UsreyMM, et al.Guidelines for implant placement to minimize impingement during activities of daily living after total hip arthroplasty. J Arthroplasty2010;25:1275–1281. Google Scholar
8 Sakalkale DP , SharkeyPF, EngK, HozackWJ, RothmanRH. Effect of femoral component offset on polyethylene wear in total hip arthroplasty. Clin Orthop2001;388:125–134. Google Scholar
9 Rubin PJ , LeyvrazPF, AubaniacJM, et al.The morphology of the proximal femur: a three-dimensional radiographic analysis. J Bone Joint Surg [Br]1992;74-B:28–32. Google Scholar
10 Sariali E , MouttetA, PasquierG, DuranteE. Three-dimensional hip anatomy in osteoarthritis: analysis of the femoral offset. J Arthroplasty2009;24:990–997. Google Scholar
11 Sariali E , MouttetA, PasquierG, DuranteE, CatoneY. Accuracy of reconstruction of the hip using computerised three-dimensional pre-operative planning and a cementless modular neck. J Bone Joint Surg [Br]2009;91-B:333–340. Google Scholar
12 Sugano N , OhzonoK, NishiiT, et al.Computed-tomography-based computer preoperative planning for total hip arthroplasty. Comput Aided Surg1998;3:320–324. Google Scholar
13 Lee YS , OhSH, SeonJK, SongEK, YoonTR. 3D femoral neck anteversion measurements based on the posterior femoral plane in ORTHODOC system. Med Biol Eng Comput2006;44:895–906. Google Scholar
14 Pasquier G , DucharneG, AliES, et al.Total hip arthroplasty offset measurement: is C T scan the most accurate option?Orthop Traumatol Surg Res2010;96:367–375. Google Scholar
15 Akbar M , AldingerG, KrahmerK, BrucknerT, AldingerPR. Custom stems for femoral deformity in patients less than 40 years of age: 70 hips followed for an average of 14 years. Acta Orthop2009;80:420–425. Google Scholar
16 Billing L . Roentgen examination of the proximal femur end in children and adolescents: a standardized technique also suitable for determination of the collum-, anteversion-, and epiphyseal angles; a study of slipped epiphysis and coxa plana. Acta Radiol Suppl1954;110:1–80. Google Scholar
17 Murphy SB , SimonSR, KijewskiPK, WilkinsonRH, GriscomNT. Femoral anteversion. J Bone Joint Surg [Am]1987;69-A:1169–1176. Google Scholar
18 Sugano N , NoblePC, KamaricE. A comparison of alternative methods of measuring femoral anteversion. J Comput Assist Tomogr1998;22:610–614. Google Scholar
19 Bland JM , AltmanDG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet1986;1:307–310. Google Scholar
20 Fackler CD , PossR. Dislocation in total hip arthroplasties. Clin Orthop1980;151:169–178. Google Scholar
21 Malik A , MaheshwariA, DorrLD. Impingement with total hip replacement. J Bone Joint Surg [Am]2007;89-A:1832–1842. Google Scholar
22 Little NJ , BuschCA, GallagherJA, RorabeckCH, BourneRB. Acetabular polyethylene wear and acetabular inclination and femoral offset. Clin Orthop2009;467:2895–2900. Google Scholar
23 Eckrich SG , NoblePC, TullosHS. Effect of rotation on the radiographic appearance of the femoral canal. J Arthroplasty1994;9:419–426. Google Scholar
24 Husmann O , RubinPJ, LeyvrazPF, de RoguinB, ArgensonJN. Three-dimensional morphology of the proximal femur. J Arthroplasty1997;12:444–450. Google Scholar
25 Clohisy JC , CarlisleJC, BeauléPE, et al.A systematic approach to the plain radiographic evaluation of the young adult hip. J Bone Joint Surg [Am]2008;90-A(Suppl):47–66. Google Scholar
26 Menschik F . The hip joint as a conchoid shape. J Biomech1997;30:971–973. Google Scholar
27 Yoshioka Y , SiuD, CookeTD. The anatomy and functional axes of the femur. J Bone Joint Surg [Am]1987;69-A:873–880. Google Scholar
28 Noble PC , AlexanderJW, LindahlLJ, et al.The anatomic basis of femoral component design. Clin Orthop1988;235:148–165. Google Scholar
29 Unnanuntana A , ToogoodP, HartD, CoopermanD, GrantRE. Evaluation of proximal femoral geometry using digital photographs. J Orthop Res2010;28:1399–1404. Google Scholar
30 Nakahara I , TakaoM, SakaiT, et al.Gender differences in 3D morphology and bony impingement of human hips. J Orthop Res2011;29:333–339. Google Scholar
31 Atkinson HD, Johal KS, Willis-Owen C, Zadow S, Oakeshott RD. Differences in hip morphology between the sexes in patients undergoing hip resurfacing. J Orthop Surg Res 2010;5:76. Google Scholar
The authors would like to thank Professor G. Aldinger and Priv.-Doz. Dr. R. Moll for clinical advice and for support in data collection.The authors thank the non-profit foundation ENDO-Stiftung, Hamburg, Germany, and the NIHR Biomedical Research Unit of Musculoskeletal Disease, Nuffield Orthopaedic Centre & University of Oxford, UK for supporting this study.
No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.
Supplementary material. Three tables, detailing i) the demographic data of the cohort, ii) the mean values for the measurements of femoral offset, diameter of the head and femoral anteversion for the entire cohort and by gender, and iii) the mean differences in femoral offset between the measurements performed on pelvis and hip anteroposterior radiographs and CT, for the entire cohort and by gender, are available with the electronic version of this article on our website www.jbjs.boneandjoint.org.uk