Ensuring the accuracy of the intra-operative orientation of the acetabular component during a total hip replacement can be difficult. In this paper we introduce a reproducible technique using the transverse acetabular ligament to determine the anteversion of the acetabular component. We have found that this ligament can be identified in virtually every hip undergoing primary surgery. We describe an intra-operative grading system for the appearance of the ligament. This technique has been used in 1000 consecutive cases. During a minimum follow-up of eight months the dislocation rate was 0.6%. This confirms our hypothesis that the transverse acetabular ligament can be used to determine the position of the acetabular component. The method has been used in both conventional and minimally-invasive approaches

Orientation of the native acetabular plane as defined by the transverse acetabular ligament (TAL) and the posterior labrum was measured intra-operatively using computer-assisted navigation in 39 hips. In order to assess the influence of alignment on impingement, the range of movement was calculated for that defined by the TAL and the posterior labrum and compared with a standard acetabular component position (abduction 45°/anteversion 15°). With respect to the registration of the plane defined by the TAL and the posterior labrum, there was moderate interobserver agreement (r = 0.64, p < 0.001) and intra-observer reproducibility (r = 0.73, p < 0.001). The mean acetabular component orientation achieved was abduction of 41° (32° to 51°) and anteversion of 18° (−1° to 36°). With respect to the Lewinnek safe zone (abduction 40° ±10°, anteversion 15° ±10°), 35 of the 39 acetabular components were within this zone. However, there was no improvement in the range of movement (p = 0.94) and no significant difference in impingement (p = 0.085). Alignment of the acetabular component with the TAL and the posterior labrum might reduce the variability of acetabular component placement in total hip replacement. However, there is only a moderate interobserver agreement and intra-observer reliability in the alignment of the acetabular component using the TAL and the posterior labrum. No reduction in impingement was found when the acetabular component was aligned with the TAL and the posterior labrum, compared with a standard acetabular component position

Abstract. Optimal acetabular component position in Total Hip Arthroplasty is vital for avoiding complications such as dislocation and impingement, Transverse acetabular ligament (TAL) have been shown to be a reliable landmark to guide optimum acetabular cup position. Reports of iliopsoas impingement caused by acetabular components exist. The Psoas fossa (PF) is not a well-regarded landmark for Component positioning. Our aim was to assess the relationship of the TAL and PF in relation to Acetabular Component positioning. A total of 22 cadavers were implanted on 4 occasions with the an uncemented acetabular component. Measurements were taken between the inner edge of TAL and the base of the acetabular component and the distance between the lower end of the PF and the most medial end of TAL. The distance between the edge of the acetabular component and TAL was a mean of 1.6cm (range 1.4–18cm). The distance between the medial end of TAL and the lowest part of PF was a mean of 1.cm (range 1,3–1.8cm) It was evident that the edge of PF was not aligned with TAL. Optimal acetabular component position is vital to the longevity and outcome following THA. TAL provides a landmark to guide acetabular component position. However we feel the PF is a better landmark to allow appropriate positioning of the acetabular component inside edge of the acetabulum inside the bone without exposure of the component rim and thus preventing iliopsoas impingement at the psoas notch and resultant groin pain

Background. The transverse acetabular ligament (TAL) has been described as an anatomic landmark to guide in the positioning of the acetabular component during total hip arthroplasty. On plain films, the radiographic teardrop (RT) has similarly been utilized as a measure of appropriate cup positioning. The goal of this study is to quantify the distance and location between the anatomic TAL and RT landmarks to aid in the positioning of acetabular component. Methods. Sixteen randomly selected cadaveric pelvises (eight male, eight female) underwent dissection. Radiographic markers were placed bilaterally at the anteromedial insertions of the TAL, and true anteroposterior (AP) pelvic radiographs of the cadavers were obtained. Distances between the markers and the lateral borders of the RT were measured. Results. The mean distance between the anteromedial insertion of the TAL and the lateral border of the RT in the male specimens was 11.8 [99% CI, 11.4 to 12.2] mm. In the female specimens, the TAL to RT distance was shorter, with a mean of 8.4 [99% CI, 7.2 to 9.6] mm. There was a statistically significant difference between male and female cadavers (p<0.01). Conclusions. The distance between the RA and TAL differs between males and females. Understanding the distance between these anatomic and radiographic landmarks should aid surgeons in obtaining a more accurate degree of acetabular component medialization, and can serve as a guide to minimize over-medialization in order to achieve more accurate and reproducible placement of acetabular components during a total hip arthroplasty

Introduction: A critical determinant of early dislocation following total hip arthroplasty (THA) is correct positioning of the acetabular component. This challenging aspect of THA has not been lessened by the introduction of more minimally invasive techniques. In this paper we introduce a simple and reproducible technique, which uses the transverse acetabular ligament (TAL) to determine cup orientation. We have used this technique as the sole method of cup orientation in our last 1000 consecutive primary total hips. Methods: One thousand consecutive patients were studied in order to determine the prevalence of early dislocation (within 3 months) following acetabular component placement determined by reference to the transverse ace-tabular ligament. All patients underwent primary total hip arthroplasty via a posterolateral approach with a posterior repair. Results: At a minimum follow-up of 9 months (range 9–39 months) 6 of the 1000 hips (0.6%) had dislocated. Conclusion: Although multiple factors are known to contribute to this rate correct placement of the acetabular component is critical. As our results compare favourably with other published series where a posterior repair has been performed by extrapolation we feel that that the TAL does provide an acceptable method of determining cup orientation. The fact that it is independent of patient position on the table and is easy to locate with a minimally invasive approach makes it an attractive method

Abstract. Background. Optimal acetabular component position in Total Hip Arthroplasty is vital for avoiding complications such as dislocation, impingement, abductor muscle strength and range of motion. Transverse acetabular ligament (TAL) and posterior labrum have been shown to be a reliable landmark to guide optimum acetabular cup position. There have been reports of iliopsoas impingement caused by both cemented and uncemented acetabular components. Acetabular component mal-positioning and oversizing of acetabular component are associated with iliopsoas impingement. The Psoas fossa (PF) is not a well-regarded landmark to help with Acetabular Component positioning. Our aim was to assess the relationship of the TAL and PF in relation to Acetabular Component positioning. Methods. A total of 12 cadavers were implanted with the an uncemented acetabular component, their position was initially aligned to TAL. Following optimal seating of the acetabular component the distance of the rim of the shell from the PF was noted. The Acetabular component was then repositioned inside the PF to prevent exposure of the rim of the Acetabular component. This study was performed at Smith & Nephew wet lab in Watford. Results. Out of the twelve acetabular components that were implanted parallel to the TAL, all had the acetabular rim very close or outside to the psoas notch with a potential to cause iliopsoas impingement. Alteration of the acetabular component position was necessary in all cadavers to inside the PF to prevent iliopsoas impingement. It was evident that the edge of PF was not aligned with TAL. Conclusion. Optimal acetabular component position is vital to the longevity and outcome following THA. TAL provides a landmark to guide acetabular component position. We feel the PF is a better landmark to allow appropriate positioning of the acetabular component inside bone without exposure of the component rim and thus preventing iliopsoas impingement at the psoas notch. Declaration of Interest. (b) declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported:I declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project

Purpose of the study: Anteversion of the cup during total hip arthroplasty (THA) is crucial for preventing the risk of dislocation. Interest has recently focused on an anatomic element often observed in the operative field during hip surgery: the transverse acetabular ligament (TAL). The TAL has become a landmark both for conventional procedures (Beverland) and for computed-assisted surgery. The purpose of this original research was to study the anteversion of the TAL in relation to the anterior pelvic plane in order to determine whether it could be a valid landmark for positioning the cup using the Lewinnek criteria (35±20° anteversion according to the Murray definition). Material and methods: Eight laboratory cadavers (three male, five female, mean age 82±3.3 years) were dissected; the pelvis was removed. Fifteen fresh healthy hips (free of trauma or degenerative disease) were also used for the study. The orientation of the peri-acetabular structures was measured with the probe of the BrainLab. ®. navigation system and the Motion Analysis. ®. system (Santa Rosa. CA) at the laboratory of biomechanics and biomechanical shocks (INRETS, Bron). The Lewinnek reference plane (anterior pelvic plane) was defined from the anterosuperior iliac spines and the pubic tubercles. Results: The anatomic version of the TAL varied from −8 to +13.3 (mean 1.9); the anatomic version of the horns of the semilunate surface from −12.2 to +14 (mean 3); for the labrum the figures were +17.4 to +41.8 (mean 26.63). Anteversion of the TAL and the horns were well correlated (r=0.8) significantly (p=0.001). Discussion: There is no other study concerning the anatomic orientation of the TAL, the horns and the labrum. Archbold was the first to consider the TAL (1000 cases, posterolateral access, 28 mm head) as a reliable constant landmark for positioning the cup (0.6% dislocation). In our study, the anatomic version of the TAL was found outside the safety zone of Lewinnek. This is a supplementary argument for questioning the reliability of the Lewinnek criteria based solely on nine cases of dislocation and criticised by several authors (non-specific for each patient). Conclusion: Anteversion of the labrum is situated within this safety zone. The TAL does not position the cup in the Lewinnek safety zone, which remains controversial

Background. Cup inclination is a major factor in the success of a total hip replacement. An open cup position can lead to dislocation or increased wear from rim loading and a closed cup position lead to impingement against the femoral neck or psoas. Although the ideal inclination for cup position is recommended as between 40 and 45 degrees, accurate positioning of the implant might be influenced by pelvic flexion and movement of the patient's pelvis during the procedure. We wanted to examine if the transvers acetabular ligament (TAL) could be used to determine cup inclination intra-operatively. Methods. 16 hips from 9 cadaveric specimens were used for the study. A computer navigation system (Brain lab) was used to measure and document the exact inclination and version of the acetabular trial component in three positions: flush with the transvers acetabular ligament (TAL), with the rim of the cup 5 mm from the TAL in a cranial direction and with the rim of the cup 5 mm caudally displaced. Statistical analysis of the results was performed by the Department of Biostatistics. Findings. With the cup positioned flush with the TAL, the average version was 43 degrees (range 37 to 47 degrees.) When there was a 5 mm gap between the TAL and the cup the average inclination was 28 degrees (21 to 35 degrees.) When the cup was opened so it covered the TAL by 5 mm the average inclination increased to 64 degrees (55 to 75 degrees.) The average anteversion angle was 18 degrees (range 15 to 25 degrees.). Conclusion. We found the transverse acetabular ligament to be an accurate landmark for positioning of the femoral implant as far as version and inclination was concerned. We recommend positioning the acetabular component flush with the TAL as cup inclination was shown to be ideal in all cases when we adhered to that principle. NO DISCLOSURES

Introduction. Malalignment of cup in total hip replacement (THR) increases rates of dislocation, impingement, acetabular migration, pelvic osteolysis, leg length discrepancy and polyethylene wear. Many surgeons orientate the cup in the same anteversion and inclination as the inherent anatomy of the acetabulum. The transverse acetabular ligament (TAL) and acetabular rim can be used as a reference. No study has yet defined the exact orientation of the TAL. The aim of this study was to describe the orientation of acetabular margin and compare it with TAL orientation. Materials and Methods. Sixty eight hips with osteoarthritis undergoing THR with computer navigation were investigated. Anterior pelvic plane was registered using anterior superior iliac spines and pubic symphysis. Orientation of the natural acetabulum as defined by the acetabular rim with any osteophytes excised was measured. Since TAL is a rectangular band like structure, three recordings were done for each corresponding to the outer middle and inner margin of the band. All the readings were given by software as radiological anteversion and inclination. Results. All patients were Caucasian, 30 males and 38 females with mean age 67.4 years (SD 9.6) and BMI 30 (SD 5). Inclination was 54.7(SD7.9), 53(SD6.9), 47.5(SD6.8), 42.1(6.7) and anteversion 5.7(SD8.7), 5.4(SD9.9), 9.7(SD9.6), 13.5(SD9.4) for acetabular rim, outer, middle and inner borders of the TAL respectively. For inclination TAL outer border was not significantly different to acetabular rim (mean difference 1.7°, 95%CIs −0.2° to 3.6°, p=0.082) but the middle (mean difference 7.3°, 95%CIs 5.6° to 8.9°) and inner (mean difference 12.6°, 95%CIs 11.0° to 14.2°) borders were (both p<0.001). For anteversion TAL outer border was not significantly different to acetabular rim (mean difference 0.2°, 95%CIs −1.3° to 1.8°, p=0.758) but the middle and inner borders were (mean difference −4.0° 95%CIs −5.5° to −2.5° and −7.9°, 95%CIs −9.6° to −6.1° respectively, both p<0.001). Anteversion for males was significantly lower than females with a mean difference of 4 for the rim and 5.7, 4.8 and 5.1 for the TAL outer, middle and inner margins respectively. Overall 57,53,40&26 of 68 patients had a combined inclination and anteversion of the native acetabulum that fell outside the “safe zone” of Lewinnek with acetabular rim, outer, middle and inner margins of TAL respectively. Compared to Lewinnek safe zones for inclination TAL inner margin performed best with 14.7% outliers and acetabular rim performed worst with 72% outliers. For anteversion TAL inner margin performed best with 25% outliers while outer margin of TAL performed worst with 39.7% outliers. Conclusion. Orientation of the acetabulum differs a lot between individuals. The TAL middle and inner margins differ in orientation as compared to acetabular rim and TAL outer border. TAL inner border provides the best bet for placing the cup in Lewinnek's safe zone. When using the natural acetabular orientation or TAL as a guide, it should not be assumed this will orientate the cup in Lewinnek safe zone although the validity of safe zones itself is questionable. Variation between patients must be taken into account and the difference between males and females, particularly in terms of anteversion, should be considered

Introduction. The aim of this study was to assess the accuracy of aligning the cup with the transverse acetabular ligament (TAL) in total hip arthroplasty (THA) and the reproducibility of this procedure by using computer-assisted navigation. Methods. Between January 2011 and March 2012, 75 patients (81 hips) underwent primary THA using the posterolateral approach at our hospital. We excluded 4 hips with a history of pelvic osteotomy; thus, the study included 77 hips. We measured the anatomical anteversion of the TAL intraoperatively by aligning the inferomedial rim of the cup trial with the TAL using computer-assisted navigation. We set the abduction to 45° at measure of the anteversion of the TAL. Measurements for each hip were independently performed thrice by 2 surgeons chosen among 1 expert and 6 non-experts. The surgeon performing the measurement was blinded during this process; the navigation screen was turned away from the surgeon's field of view. Anatomical inclination and anteversion were measured with reference to the functional pelvic plane. The intraclass correlation coefficient (ICC) was used to assess intra- and inter-observer reliability. The mean value of all 6 measurements was used to determine the anteversion of the TAL in each hip. Results. The TAL was identified in 83% of the cases (64 of 77 hips). Intra-observer reliability was high for both the expert surgeon (ICC(1.1) = 0.851) and the non-expert surgeons (ICC(1.1) = 0.825). Inter-observer reliability was moderate (ICC(2.1) = 0.452). The mean difference in the anatomical anteversion measured by 2 surgeons was 7.0° (5.3°) (range, 0.3–21.3°). The mean anatomical anteversion of the TAL was 20.9° (7.0°) (range, 9.0–48.3°). Discussion and Conclusions. Recently, reports have suggested that the TAL can be used as a reference for determining a patient's native acetabular anteversion; the position of the cup can then be customized so that the face of the acetabular component is parallel to the TAL. We measured the anatomical anteversion of the cup trial aligned with the TAL using computer-assisted navigation and assessed the reproducibility of the alignment. Intra-observer reliability was high, and each surgeon was able to align the cup according to his target for of the TAL anteversion. However, inter-observer reliability was only moderate. This is because the TAL is a short ligament with some thickness, and the methods employed to align the cup trial with the TAL may differ among surgeons. The smallest anteversion of the TAL was 9°, and retroversion was not observed in any of the cases. Therefore, in our opinion, the TAL is useful as a reference for not positioning the cup in retroversion. However, in some cases with an excessive posterior pelvic tilt, the anteversion of the TAL may have been excessive and not necessarily optimal. Therefore, aligning the cup with TAL may not be the ideal method for all cases

Introduction: Autopsy findings (Jensen and Lauritzen 1976, Catterall et al. 1982) as well as own MRI studies (Lange et al. 1996) indicate that in Perthes’ disease there is an early cartilaginous enlargement of the femoral head. Lack of concomitant acetabular enlargement will lead to loss of containment and subluxation. We divided the transverse acetabular ligament (TAL) to promote expansion of the acetabulum for prevention of femoral head extrusion and loss of containment. Material and Methods: We report 13 patients with Perthes’disease belonging to Catterall group III or IV. The operation was performed when MRI showed a labrum lift near horizontal position indicating risk of loss of containment (Meiss 2001). There was an average cartilaginous head enlargement of 11 % in comparison to the uneffected side as measured by the Maximum Oblique Diameter. Division of the TAL was performed through an antero-medial approach (Ludloff 1913, Wein-stein 1993). A window of about 1,5 x 1 cm was created in the capsule which was left open. The TAL was divided but not removed. In all cases strict non-weightbearing was imposed postoperatively (use of a wheel chair and crutches) until well into the regeneration phase. The average period of non-weight-bearing was 1 year and 10 months. An abduction pillow was worn at night. Results: The results after an average follow-up of 4,2 years (range 2,6 -5,3 years) were evaluated according to Stulberg (1981) and Catterall (1982) with emphasis on the radiographic appearance (sphericity of the femoral head, joint congruity, containment [acetabulum head index], articulo-trochanteric distance). The result was excellent in 2 cases, good in 8, satisfactory in 2, and poor in 1 case. Two patients had a bad compliance for non-weightbearing and underwent additional bony procedures. The outcome was satisfactory and poor. Conclusions: Our data indicate that the combination of the division of the TAL and a strict conservative treatment gives very satisfactory results in Catterall group III and IV cases that present with signs of risk of loss of containment on MRI

Background: The positioning of the acetabular component is of critical importance in total hip arthroplasty. Due to the orientation of the acetabulum and limitations of observation imposed at the operative site mal-positioning is common. We believe that by utilising the transverse acetabular ligament (TAL) and acetabular labrum, we are able to anatomically position our cup. In this study, we evaluate the correlation between placement of the acetabular component by reference to the TAL and the acetabular labrum with the taught safe zones for cup placement. Method: 7 embalmed hips were studied. Following disarticulation the labrum and TAL were digitised and their plane was calculated. Orientation of cup placement in this plane was calculated from a pre-dissection pelvic CT. Results: The plane of the labrum/TAL varied between 5–26° of anteversion and 32–59° of inclination. Interob-server differences in acetabular cup placement based on the TAL/labral plane indicate reasonable coherence. Almost all components were inside the documented “safe zone” 0–40° of anteversion and 30–55° of inclination of placement. Conclusion: The acetabular labrum and TAL form a plane that reflects the documented “safe zones” for acetabular component placement. We feel that this plane allows a surgeon to determine optimal patient specific acetabular component placement, irrespective of patient position

Current methods for restoring or preserving limb length following total hip arthroplasty are anatomically inaccurate, as they do not consider acetabular and femoral height independently. In order to address this, we present and evaluate a technique that uses the transverse acetabular ligament to control the vertical height of the acetabular component and a caliper that controls the vertical placement of the femoral component within the femoral canal. Limb lengths were measured in 200 patients who had undergone primary total hip arthroplasty using this technique. Using this method, 94% had a post-operative limb length inequality that was 6 mm or less when compared to the normal side (average +0.38 mm). The maximum measured limb length inequality was ± 8 mm

Published data has shown that only 45% of acetabular components were in an acceptable position, where positioning was determined clinically by the surgeon intra-operatively. The aim of this study is to assess the accuracy of cup orientation, using computer tomography (CT), when the TAL is used as the intra-operative guide.

In this prospective study, the TAL was used as the anatomical reference for positioning the cup. The TAL was graded 1 to 4 based on visibility of the ligament. The version and abduction angles were estimated clinically and recorded by the surgeon after insertion of the cup. Post-operatively the true orientation of the cup was measured using CT. Statistical analyses were carried out to calculate the difference between the intra-operative estimation of cup orientation and the true cup position as measured by CT. Ethical approval was granted and informed consent was obtained for all the patients.

Forty-eight hips have been studied to date. The TAL was easily identifiable in the majority of cases. Overall, the cup version was under-estimated by the surgeon when the TAL was utilized as the anatomical landmark. The true mean acetabular component version was 26.5 degrees [range from 11 to 41 degrees]. The true mean abduction angle was 43.6 degrees [range from 35 to 55 degrees]. The mean difference between surgeon estimation and CT measurement for cup version was 4 degrees of underestimation [range from 14 degrees of underestimation to 11 degrees of overestimation]. The mean difference for abduction angle was 0.1 degrees [range from 14 degrees of underestimation to 10 degrees of overestimation]. When using TAL as an intra-operative guide, 64% of acetabular components were within the target range of 15 to 30 degrees of anteversion, as measured by CT, compared to 45% in previously published study (Wines, A & McNicol, D, J. Arthroplasty, 2006).

TAL improves the accuracy of acetabular component version, when utilized as an anatomical landmark during cup insertion in primary total hip arthroplasty. It is reliable and easily identifiable in the majority of cases.

It has recently been reported that the transverse acetabular ligament (TAL) is helpful in determining the position of the acetabular component in total hip replacement (THR). In this study we used a computer-assisted navigation system to determine whether the TAL is useful as a landmark in THR. The study was carried out in 121 consecutive patients undergoing primary THR (134 hips), including 67 dysplastic hips (50%). There were 26 men (29 hips) and 95 women (105 hips) with a mean age of 60.2 years (17 to 82) at the time of operation. After identification of the TAL, its anteversion was measured intra-operatively by aligning the inferomedial rim of the trial acetabular component with the TAL using computer-assisted navigation. The TAL was identified in 112 hips (83.6%). Intra-observer reproducibility in the measurement of anteversion of the TAL was high, but inter-observer reproducibility was moderate.

Each surgeon was able to align the trial component according to the target value of the angle of anteversion of the TAL, but it was clear that methods may differ among surgeons. Of the measurements of the angle of anteversion of the TAL, 5.4% (6 of 112 hips) were outliers from the safe zone.

In summary, we found that the TAL is useful as a landmark when implanting the acetabular component within the safe zone in almost all hips, and to prevent it being implanted in retroversion in all hips, including dysplastic hips. However, as anteversion of the TAL may be excessive in a few hips, it is advisable to pay attention to individual variations, particularly in those with severe posterior pelvic tilt.

Cite this article: Bone Joint J 2014;96-B:306–11.

The orientation of the acetabular component can influence both the short- and long-term outcomes of total hip replacement (THR). We performed a prospective, randomised, controlled trial of two groups, comprising of 40 patients each, in order to compare freehand introduction of the component with introduction using the transverse acetabular ligament (TAL) as a reference for anteversion. Anteversion and inclination were measured on pelvic radiographs. With respect to anteversion, in the freehand group 22.5% of the components were outside the safe zone versus 0% in the transverse acetabular ligament group (p = 0.002). The mean angle of anteversion in the freehand group was 21° (2° to 35°) which was significantly higher compared with 17° (2° to 25°) in the TAL group (p = 0.004). There was a significant difference comparing the variations of both groups (p = 0.008). With respect to inclination, in the freehand group 37.5% of the components were outside the safe zone versus 20% in the TAL group (p = 0.14). There was no significant difference regarding the accuracy or variation of the angle of inclination when comparing the two groups. . The transverse acetabular ligament may be used to obtain the appropriate anteversion when introducing the acetabular component during THR, but not acetabular component inclination. . Cite this article: Bone Joint J 2014;96-B:312–18

Despite a lack of long-term follow-up, there is an increasing trend towards using femoral heads of large diameter in total hip replacement (THR), partly because of the perceived advantage of lower rates of dislocation. However, increasing the size of the femoral head is not the only way to reduce the rate of dislocation; optimal alignment of the components and repair of the posterior capsule could achieve a similar effect. . In this prospective study of 512 cemented unilateral THRs (Male:Female 230:282) performed between 2004 and 2011, we aimed to determine the rate of dislocation in patients who received a 22 mm head on a 9/10 Morse taper through a posterior approach with capsular repair and using the transverse acetabular ligament (TAL) as a guide for the alignment of the acetabular component. The mean age of the patients at operation was 67 years (35 to 89). The mean follow-up was 2.8 years (0.5 to 6.6). Pre- and post-operative assessment included Oxford hip, Short Form-12 and modified University of California Los Angeles and Merle D’Aubigne scores. The angles of inclination and anteversion of the acetabular components were measured using radiological software. There were four dislocations (0.78%), all of which were anterior. In conclusion, THR with a 22 mm diameter head performed through a posterior approach with capsular repair and using the TAL as a guide for the alignment of the acetabular component was associated with a low rate of dislocation. Cite this article: Bone Joint J 2014;96-B:1202–6

Introduction. Acetabular component orientation is an important determinant of outcome following total hip arthroplasty (THA). Although surgeons aim to achieve optimal cup orientation, many studies demonstrate their inability to consistently achieve this. Factors that contribute are pelvic orientation and the surgeon's ability to correctly orient the cup at implantation. The goal of this study was to determine the accuracy with which surgeons can achieve cup orientation angles. Methods. In this in vitro study using a calibrated left and right sawbone hemipelvis model, participants (n=10) were asked to place a cup mounted on its introducer giving different targets. Measurements of cup orientation were made using a stereophotogrammetry protocol to measure radiographic inclination and operative anteversion (OA). A digital inclinometer was used to measure the intra-operative inclination (IOI) which is the angle of the cup introducer relative to the floor. First, the participant stated his or her preferred IOI and OA and positioned the cup accordingly. Second, the participant had to position the cup parallel to the anteversion of the transverse acetabular ligament (TAL). Third, the participant had to position the cup at IOI angles of 35°, 40° and 45°. Fourth, the participant used the mechanical alignment guide (45° of IOI and 30° of OA) to orient the cup. Each task was analysed separately and subgroup analysis included left versus right side and hip surgeons versus non-hip surgeons. Results. For the first task, hip surgeons preferred smaller IOI and larger OA than non-hip surgeons, but there was no significant difference in accuracy between both groups. When aiming for TAL, both surgeon groups performed similar, but accuracy on the non-dominant side was significantly better compared with the dominant side (mean deviation 0.6° SD 2.4 versus −2.6° SD 2.3) (p=0.004). When aiming for a specific IOI target of 35°, 40° or 45°, non-hip surgeons outperformed hip surgeons (mean deviation form target IOI 1.9° SD 2.7 versus −3.1° SD 3.8) (p<0.0001) with less variance (p=0.03). Contrary to version, accuracy on the dominant side was significantly better compared with the non-dominant side (mean deviation −0.4° SD 3.4 versus −2.1° SD 4.8). When using a mechanical guide, surgeons performed similar (0.6° SD 1.2 versus −0.4° SD 2.1 for inclination p=0.11 and −0.5° SD 2.6 versus −1.8° SD 3.3 for version p=0.22) and these values did not differ significantly from the actual IOI and OA of the mechanical guide. When using a mechanical guide, there was no difference in accuracy between the dominant and non-dominant side. Conclusion. There was no difference in accuracy between hip surgeons and non-hip surgeons when they aimed for their preferred IOI and OA or used a mechanical guide. When aiming for a specific IOI target, non-hip surgeons outperformed hip surgeons. Hip surgeons overestimate IOI and underestimate OA, presumably because this helps to achieve the desired radiographic cup orientation. Regarding accuracy, the non-dominant side was better for version and the dominant side for inclination. When aiming for a specific IOI and OA target, using a mechanical guide is significantly better than freehand cup orientation

Ideal cup positioning remains elusive both in terms of defining and achieving target. Our aim is to restore original anatomy by using the Transverse Acetabular Ligament (TAL). In the normal hip TAL and labrum come beyond the equator of the femoral head therefore if the definitive cup is positioned such that: It is cradled by the TAL; Is ideally no more than 4mm greater in diameter than the original femoral head; Sits just deep to the plane of TAL and labrum (this means that normally we leave the fat pad intact and do not ream down to the true floor). That should restore joint center in terms of height and offset. If the face of the cup is then positioned parallel and just deep to the TAL and psoas groove that should restore original version. We still use TAL for version in dysplasia because we believe the TAL and labrum compensate for any underlying bony abnormality. However in dysplasia the TAL and labrum fall short of original femoral head equator and therefore in such cases we ream down to the true floor if necessary and use a cup which is often smaller than the original head. Inclination represents a greater challenge and TAL should not be used as an aid to inclination. Our research has shown that errors in postoperative x-ray inclination above 50 degrees are generally caused by errors in patient positioning when in lateral decubitus. Consequently great care needs to be taken when positioning the patient

Introduction. Cup positioning in total hip arthroplasty (THA) is an important variable for short and long term durability of any hip implant. This novel method utilises internal and external bony landmarks, and the transverse acetabular ligament for positioning the acetabular component. Methods. The cup is placed parallel and superior to the transverse ligament and inside the anterior wall notch of the true acetabulum, and then adjusted for femoral version and pelvic tilt, fixed obliquity, and transverse rotational deformity based on weight bearing pre-operative radiographs. Seventy consecutive THRs (68 patients) were performed using the above technique. The cup radiographic and functional anteversion and abduction angle were measured on post-operative weight bearing pelvic radiographs using EBRA software. Results. The mean follow-up was 8.1 ± 2.4 months (4.3 – 11.8 months). There were no dislocations. The mean anteversion and abduction angle was 41.8 degrees ± 4.6 degrees and 18.5 degrees ± 4.4 degrees, respectively. In 3 hips, the radiographic abduction angle was slightly outside the safe zone of Lewinnek as measured based on the inter-teardrop line. However, when using a weight bearing AP pelvis radiograph to measure functional abduction angle using a horizontal line as a reference, they were all within the normal range. Discussion and Conclusion. The proposed technique utilises intra- and extra-articular bony landmarks, allows for adjustment for lumbosacral angle, abnormal femoral anteversion, and excessive acetabular version. The proposed technique is a reproducible and accurate method for cup placement with posterior exposure