Cup Abduction Angle Research Articles

The Importance of Proper Acetabular Component Positioning and the Challenges to Achieving It

The importance of proper cup placement cannot be overemphasized, regardless of the bearing material or diameter. Unsatisfactory acetabular component position has been associated with instability, increased wear, and pain. Pelvic, acetabular, and femoral anatomy are all variable, so it is illogical to have the same fixed target position for all patients. The hip arthroplasty surgeon actually faces 2 challenges: (1) determining the desired acetabular component position for each patient (the target), and (2) how to reasonably obtain that position in surgery (hitting the target). An abduction angle of 40° ± 10° is generally satisfactory. Anteversion is more complex. The desired amount of anteversion is influenced by (a) the amount of femoral anteversion and (b) the cup abduction angle. A combined anteversion of 25° ± 10° is generally satisfactory. A combination of internal and external landmarks can be used to assess the relative component position. Routine evaluation of intraoperative range of motion is an additional check. When in doubt, we try to obtain a quality intraoperative image.

Intra- and Intersurgeon Variability in Image-free Navigation System for THA

In image-free navigation systems, cup orientation is determined in the pelvic coordinate by registration of bony landmarks. While the value of navigation relates primarily to the reliability and accuracy of cup placement, the reliability of registration plays a role in cup placement. We therefore examined intra- and intersurgeon variability in registration and the distance between registration points in each bony landmark. Thirty-seven THAs were performed in the lateral position and 15 THAs in the supine position. The cup was fixed using a navigation system. The registration was repeated two more times by operator and assistant, and the intra- and intersurgeon variability of cup abduction angle and anteversion was analyzed by ICC (intraclass correlation coefficients). In 25 hips, the distance between intrasurgeon registration points and between intersurgeon registration points in each landmark were calculated. The ICC in the lateral position ranged between 0.59 and 0.81, and between 0.85 and 0.95 in the supine position. The ICCs of cup abduction angle for the intra- and intersurgeon variability were 0.92 and 0.95 for the supine position and 0.65 and 0.59 for the lateral position. Those of anteversion were 0.93, 0.85, and 0.81, 0.72, respectively. The variability in locating the ASIS in the lateral position was greater than that in the supine position. The variability of registration points depended on bony landmarks and patient position but the range of variability we found would not likely result in a large variability in cup placement.

Mathematical evaluation of jumping distance in total hip arthroplasty

Background and purpose The jumping distance (JD) is the degree of lateral translation of the femoral head center required before dislocation occurs. The smaller the distance, the higher the theoretical risk of dislocation. The aim of our study was to evaluate this jumping distance and its variation according to the characteristics of the implant, and also the theoretical gain in using large head diameters of above 38 mm.Methods The JD was calculated as a function of the cup ante-version and abduction angles, the head diameter, and the head offset (defined as the distance between the center of the femoral head and the cup opening plane). Head diameters of 28, 32, 36, 40, 44 and 48 mm were analyzed. The abduction angle was increased from 0° to 80° with a 10° increment. The anteversion angle was increased from 0° to 40° with a 5° increment.Results The jumping distance was found to decrease as the cup abduction angle increased (0.25 mm each 1° for 32-mm head diameter). It increased by 0.05 mm for a 1° increase in the ante-version angle. The jumping distance increased as the head diameter increased (0.4 mm each mm diameter for a 45° abduction angle). The net gain obtained by increasing the diameter, however, decreased when abduction angle increased (0.25 each mm diameter for 60° abduction). The JD decreased by 0.92 mm for each 1-mm increase in head offset, showing that head offset was the most important parameter influencing the JD.Interpretation The theoretical gain in stability obtained by using a large femoral head (above 36 mm) is negligible in cases where there is a high cup abduction angle. An increase in offset of the femoral head substantially reduces the jumping distance and it should therefore be avoided.

Hip resurfacing arthroplasty for secondary osteoarthritis after developmental dysplasia of hip

To study the clinical results of metal on metal hip resurfacing arthroplasty for developmental dysplasia patients. From March 2005 to December 2006, 34 cases of developmental dysplasia patients (Crowe I, Crowe II) were attempted to have metal on metal hip resurfacing arthroplasty. There were 29 females (32 hips), 5 males (5 hips). The average age was 45 (26 - 57) years old. Radiographic and clinical evaluations were taken at 6 weeks, 3 months, 1 year and then once a year postoperatively. The average Harris score was 35 (25 - 44). Hip flexion was 101 degrees , abduction 24 degrees , adduction 15 degrees . Three patients were turned to total hip arthroplasty during operations. Thirty-one patients (34 hips) received hip resurfacing surgery. These 31 patients were followed for average 21.4 months (12 - 33 months). The average Harris score was 94 (82 - 100) at the latest follow-up, and there was statistical difference compared with the preoperative score (P < 0.01). Hip flexion increased to 133 degrees , abduction to 48 degrees , adduction to 26 degrees . No radiolucency line was found at both acetabular and femoral sides in all the patients. The average abduction angle of acetabular cup was 43 degrees (40 degrees - 53 degrees ), and the average stem shaft angle was 139 degrees (130 degrees - 145 degrees ). The short term result is excellent. While the mid to long term results for hip resurfacing arthroplasty in developmental dysplasia patients are still looking forward, and the meticulous surgical technique and strict patient selection are the key of the good results.

In Vivo Wear Rate of the Birmingham Hip Resurfacing Arthroplasty: A Review of 10 Retrieved Components

The wear of 8 femoral and 2 acetabular components of the Birmingham Hip Resurfacing (Midland Medical Technologies Ltd, Birmingham, UK) was measured using a coordinate measuring machine (CMM5; SIP, Geneva, Switzerland). After a median in situ time of 13 months, measurements of the Birmingham Hip Resurfacing femoral heads revealed a median volumetric wear of 2.9 mm 3. In one case with a cup abduction angle of 70°, a significantly higher wear of 17.8 mm 3 was found. The 2 acetabular components showed volumetric wear of 5.4 and 27.6 mm 3 after 14 and 15 months in vivo. The wear rates of the investigated femoral components were somewhat, but not significantly, higher than the previously investigated 28-mm Metasul heads (median, 0.8 mm 3; Zimmer GmbH, Winterthur, Switzerland).

A modified two-incision technique for primary total hip arthroplasty

Background:Minimally invasive surgery can be technically demanding but minimizes surgical trauma, pain and recovery. Two-incision minimally invasive surgery allows only intermittent visualization and may require fluoroscopy for implant positioning. We describe a modified technique for primary total hip arthroplasty, using two small incisions with a stepwise approach and adequate visualization to reliably and reproducibly perform the surgery without fluoroscopy.Materials and Methods:One hundred and two patients with an average age of 60 years underwent modified two-incision minimally invasive technique for primary THA without fluoroscopy. The M/L taper femoral stem (Zimmer, Warsaw, IN) and Trilogy (Zimmer) hemispherical titanium shell, with a highly cross-linked polyethylene liner, was used. Operative time, blood loss, postoperative hospital stay, radiographic outcomes and complications were recorded.Results:The mean operating time was 77 min. The mean blood loss was 335 cc. The mean hospital stay was 2.4 days. Mean cup abduction angle was 43.8°. Mean leg length discrepancy was 1.7 mm. Thirteen patients had lateral thigh numbness and two patients had wound complications that resolved without any treatment.Conclusion:A modified two-incision technique without fluoroscopy for primary total hip arthroplasty has the advantage of preserving muscles and tendons, shorter recovery and return to function with minimal complications. Provided that the surgeon has received appropriate training, primary total hip arthroplasty can be performed safely with the modified two-incision technique.

Randomised Controlled Trial Comparing two Methods of Acetabular Cup Positioning during Total Hip Arthroplasty

Acetabular cup positioning is an important technical aspect in total hip arthroplasty. Most surgeons estimate cup abduction angle during surgery with the insertion rod position according to the patient's body anatomical landmarks or other reference points in the operating room. High acetabular component abduction angle is associated with an increased risk of dislocation, premature polyethylene wear and osteolysis. To evaluate the potential benefits of a new technique for vertical acetabular cup positioning, 100 acetabular cups were randomised to be inserted with or without an inclinometer. Abduction angles were measured on postoperative radiographs by 2 evaluators blind to the treatment group. Of the cups, 57% (27/47) were positioned within the desirable abduction angle range of 40-49 with the inclinometer, compared with 50% (27/53) by visuospatial perception (p=0.454). The proportion of cups positioned outside a safe angle range of 30-55 was low in both groups: 6% (3/47) for the inclinometer group versus 4% (2/53) for the visuospatial perception group (p=0.536). The use of an inclinometer did not significantly improve the acetabular cup abduction angle obtained by our group of surgeons when compared with visuospatial perception. Newer techniques such as navigation may be useful in further optimising cup positioning and reducing the outliers.

The Position of the Bipolar Cup Reflects the Direction of the Hip Contact Force Acting on It

We radiographically measured the bipolar cup position to analyze the direction of joint force acting on the bipolar cup. The abduction angle of the bipolar cup was measured in each radiograph taken immediately and at six 12 weeks and yearly after the operation. Radiographs in patients with weight bearing were also investigated. The results indicated that the abduction angle of the bipolar cup was 24.1° ± 11.2° immediate postoperatively and was 16.2° ± 5.1° at 6 weeks, 16.1° ± 5.1° at 3 months, and 16.2° ± 5.1° at 1 year. The cup abduction angles with weight bearing were not different from those without weight bearing and were 15.9° ± 4.9°, 16.2° ± 4.4°, and 16.1° ± 4.7° on the supine, double-legged stance, and single-legged stance radiographs, respectively. Because the position of the bipolar cup reflects the direction of loads pivoting on it, the direction of the joint force in the frontal plane acting on the bipolar prosthesis is about 16° to vertical.

Randomised controlled trial comparing two methods of acetabular cup positioning during total hip arthroplasty

Acetabular cup positioning is an important technical aspect in total hip arthroplasty. Most surgeons estimate cup abduction angle during surgery with the insertion rod position according to the patient's body anatomical landmarks or other reference points in the operating room. High acetabular component abduction angle is associated with an increased risk of dislocation, premature polyethylene wear and osteolysis. To evaluate the potential benefits of a new technique for vertical acetabular cup positioning, 100 acetabular cups were randomised to be inserted with or without an inclinometer. Abduction angles were measured on postoperative radiographs by 2 evaluators blind to the treatment group. Of the cups, 57% (27/47) were positioned within the desirable abduction angle range of 40-49 with the inclinometer, compared with 50% (27/53) by visuospatial perception (p=0.454). The proportion of cups positioned outside a safe angle range of 30-55 was low in both groups: 6% (3/47) for the inclinometer group versus 4% (2/53) for the visuospatial perception group (p=0.536). The use of an inclinometer did not significantly improve the acetabular cup abduction angle obtained by our group of surgeons when compared with visuospatial perception. Newer techniques such as navigation may be useful in further optimising cup positioning and reducing the outliers.

Obesity Does Not Affect The Accuracy of Acetabular Cup Implantation in Total Hip Replacement

Post-operative radiographs of 111 patients who had undergone cemented Total Hip Replacement performed or supervised a single surgeon (GCF) were analysed. All surgeries were performed via an anterolateral approach with patients in the lateral position. Patients were stratified according to Body Mass Index (BMI) into three groups (healthy, overweight and obese) and acetabular cup angles measured. Mean values for cup abduction angle were similar for all three groups (44.5°, 46.8°, 44.0° respectively, p=0.335). Furthermore there were no differences in mean cup anteversion between groups (11.6°, 12.2°, 10.7 respectively, p=0.665). We conclude that obesity does not influence the accuracy of cup positioning in Total Hip Replacement.

Antero-lateral minimal invasive (ALMI) approach for total hip arthroplasty technique and early results

Minimally invasive surgery represents one of the most recent techniques to have emerged within THA. In conventional THA, the incision typically measures 15-20 cm. Minimal invasive approach defined as less invasive to the skin, muscles, or bone may reduce complications and improve recovery time. A number of different approaches and methods have been described in literature. This is a prospective study describing the technique and early results of the modified antero-lateral minimal invasive (ALMI) approach and comparing our results to the results of other investigators interested in minimal invasive THA. Seventy-five consecutive primary total hip arthroplasties (cemented and cementless) were done through a modified ALMI approach (6-8 cm), in which we kept the hip abductors intact. Neither special instruments nor specially designed prostheses were needed. Minimum follow-up was 12 months. The mean Harris hip score for patients after 12-month follow-up was 90 while the mean Merle d'Abugine mean score was 16.5. Both scores reached almost the maximum values within 3 months after surgery. The cup abduction angle for 70% patients was between 35 degrees and 45 degrees. No femoral stem mal-alignment was recorded. The mean operative time of cemented prosthesis was 65 min while that of the cementless prosthesis was 55 min. No wound complications or dislocations were recorded. All the patients were allowed to weight bear in the second post-operative day and involved in an early rehabilitation program. ALMI hip approach with sparing of hip abductors is safe and gives excellent orientation for positioning of prosthesis components. It also allows early and smooth post-operative rehabilitation with fast recovery of the patient in terms of weight bearing.

Outcome of an Acetabular Design with Hydroxyapatite Coating on a Rough Substrate

Outcomes of hydroxyapatite-coated cups have not been as consistently successful as outcomes of hydroxyapatite-coated stems; therefore, we studied a newer generation acetabular design with a plasma-sprayed hydroxyapatite coating on an arc-deposited rough titanium substrate. Our objective was to determine whether clinical and radiographic outcomes would be better than reported for earlier-generation designs. Ninety consecutive hips in 85 patients implanted with this design were followed up prospectively for 2 to 5 years after surgery. Hip scores improved from a preoperative mean of 59 (+/- 12) to final followup mean of 91 (+/- 12). One hip was revised for recurrent dislocation. No hips were revised for aseptic loosening. No cup had complete continuous radiolucent lines. Incomplete lucent lines were noted on early postoperative radiographs. With followup, radiolucent line length decreased by a mean 32% and radiolucent line density increased by 9%. Mean polyethylene linear and volumetric head penetration rates were 0.17 (+/- 0.16) mm/year and 73 (+/- 70) mm/year, respectively. Head penetration rates were higher in men. No other factor (age, body weight, cup abduction, or anteversion angle) correlated with head penetration rate. Our study presents encouraging short-term results. Improvement in radiolucent lines suggests a beneficial effect of hydroxyapatite coating on osseointegration of the cup. Therapeutic study, Level IV (case series). See the Guidelines for Authors for a complete description of levels of evidence.

Effectiveness of an Acetabular Positioning Device in Primary Total Hip Arthroplasty

To evaluate the efficacy of a commercially available acetabular positioning device, we performed a prospective evaluation of 40 consecutive patients undergoing primary total hip arthroplasty. All surgery was performed by the same surgeon, in the same operating room, and on the same operating table. The acetabular positioning device was designed to place the component in 45 degrees of abduction. At 6 weeks, all radiographs were evaluated by 3 investigators not involved with the surgery. Each radiograph was evaluated by each reviewer on 3 separate occasions, blinded to the findings of the other reviewer to assess interobserver and intraobserver variability. The mean cup abduction angle was 42.1 degrees, with a range from 23 degrees to 57 degrees (SD 8.3 degrees). Intraobserver and interobserver variability were 0.2 and 0.3 degrees, respectively. The findings of this study demonstrate a wide variability in acetabular cup placement in primary total hip arthroplasty. We believe this is due to movement of the pelvis, which may occur during preparation, draping, and retracting during surgery. We feel surgeons should not rely solely on positioning devices when implanting the acetabular component in total hip arthroplasty. Identification of bone landmarks and determination of superolateral implant coverage noted on preoperative templating is advocated to improve the precision of component position.

The effect of geometry and abduction angle on the stresses in cemented UHMWPE acetabular cups – finite element simulations and experimental tests

BackgroundContact pressure of UHMWPE acetabular cup has been shown to correlate with wear in total hip replacement (THR). The aim of the present study was to test the hypotheses that the cup geometry, abduction angle, thickness and clearance can modify the stresses in cemented polyethylene cups.MethodsAcetabular cups with different geometries (Link®: IP and Lubinus eccentric) were tested cyclically in a simulator at 45° and 60° abduction angles. Finite element (FE) meshes were generated and two additional designs were reconstructed to test the effects of the cup clearance and thickness. Contact pressures at cup-head and cup-cement interfaces were calculated as a function of loading force at 45°, 60° and 80° abduction angles.ResultsAt the cup-head interface, IP experienced lower contact pressures than the Lubinus eccentric at low loading forces. However, at higher loading forces, much higher contact pressures were produced on the surface of IP cup. An increase in the abduction angle increased contact pressure in the IP model, but this did not occur to any major extent with the Lubinus eccentric model. At the cup-cement interface, IP experienced lower contact pressures. Increased clearance between cup and head increased contact pressure both at cup-head and cup-cement interfaces, whereas a decreased thickness of polyethylene layer increased contact pressure only at the cup-cement interface. FE results were consistent with experimental tests and acetabular cup deformations.ConclusionFE analyses showed that geometrical design, thickness and abduction angle of the acetabular cup, as well as the clearance between the cup and head do change significantly the mechanical stresses experienced by a cemented UHMWPE acetabular cup. These factors should be taken into account in future development of THR prostheses. FE technique is a useful tool with which to address these issues.

Clinical Outcome of Current Generation Hydroxyapatite Coated Acetabular Design

Hydroxyapatite (HA) coatings for femoral stems have been successful in short- to intermediate-term follow up. However, the outcomes of HA-coated cups have not been as successful as comparable non-HA-coated designs. This has been attributed to component design and to poor quality HA-coating processes. This is a clinical outcome study of a newer generation acetabular design with an improved plasma-sprayed HA-coat. Ninety consecutive primary total hip replacements implanted with a HA-coated metal-backed cup were prospectively followed up for 2–5 years after surgery. Mean age was 69 years, male:female ratio was 36:47; mean-weight was 77 kg. Polyethylene wear was measured on digitized images. Component position, component migration, and implant-bone interface radiolucent lines were analyzed by computerized image analysis. Hip-scores improved from preoperative mean 59(±12) to a final follow-up mean of 91(±12). One hip was revised for recurrent dislocation. No hips were revised for aseptic loosening. Mean acetabular abduction angle was 46°(±8.5) and anteversion was 18°(±8.7). Incomplete lucent lines were noted on early postoperative radiographs in 5% of the cases. With ongoing follow up radiolucent line thickness decreased by mean 13% and radiolucent line density increased by 11%. No cup had complete continuous radiolucent lines or lysis. Mean polyethylene linear and volumetric wear rates were 0.14(± 0.15) mm/year and 73(± 70) mm3/year respectively. Wear rates were higher in males (p&lt;0.03). No other factor (age, body weight, cup abduction or anteversion angle) correlated significantly with wear rate. This study presents encouraging short-term results. The improvement in radiolucent lines suggests a beneficial effect of the HA-coat on bone ingrowth. None of the cups loosened or migrated. Wear rates were comparable to published reports of successful non-HA-coated cups. Long-term follow up is ongoing to corroborate these early findings.

Measurements of acetabular cup position and pelvic spatial orientation after total hip arthroplasty using computed tomography/radiography matching

This study presents a clinical validation of postoperative measurements of acetabular cup alignment following total hip arthroplasty (THA). The methodology was based on concurrent anatomic three-dimensional (3D) measurements of both the acetabular cup alignment and pelvic orientation, using an original CT/X-ray matching algorithm named Xalign. The subjects were 19 patients who had undergone bilateral THA using CT-based surgical navigation. All patients had postoperative pelvic CT scans and multiple antero-posterior (AP) pelvic X-rays. Using a proprietary software algorithm, the X-rays included in the study were matched with the corresponding postoperative CT scans. The goal of this method was to allow 3D anatomic pelvic and acetabular measurements on two-dimensional AP X-rays. The postoperative cup abduction, version and pelvic flexion angles were determined in three different ways: using CT images directly, applying the Xalign method, and finally by performing conventional (abduction only) measurements on AP pelvic X-rays. The cup orientation measured on CT images was taken as the ground truth. The Xalign measurement errors were defined as the difference between the CT cup values and those obtained by applying the matching method. The mean cup abduction error was 0.85° ± 1.3° (± standard deviation) and the mean version error was 0.01° ± 1.99°. Conventionally measured cup abduction ranged from 44° to 62° and correlated significantly (p = 0.001, r = −0.5) with pelvic flexion angle, proving the linear negative correlation between pelvic flexion and the error in conventional radiographic cup measurements. The Xalign method offered reasonable accuracy for cup orientation, and allowed cup and pelvic 3D anatomic measurements at different times.

Measurements of acetabular cup position and pelvic spatial orientation after total hip arthroplasty using computed tomography/radiography matching

This study presents a clinical validation of postoperative measurements of acetabular cup alignment following total hip arthroplasty (THA). The methodology was based on concurrent anatomic three-dimensional (3D) measurements of both the acetabular cup alignment and pelvic orientation, using an original CT/X-ray matching algorithm named Xalign. The subjects were 19 patients who had undergone bilateral THA using CT-based surgical navigation. All patients had postoperative pelvic CT scans and multiple antero-posterior (AP) pelvic X-rays. Using a proprietary software algorithm, the X-rays included in the study were matched with the corresponding postoperative CT scans. The goal of this method was to allow 3D anatomic pelvic and acetabular measurements on two-dimensional AP X-rays. The postoperative cup abduction, version and pelvic flexion angles were determined in three different ways: using CT images directly, applying the Xalign method, and finally by performing conventional (abduction only) measurements on AP pelvic X-rays. The cup orientation measured on CT images was taken as the ground truth. The Xalign measurement errors were defined as the difference between the CT cup values and those obtained by applying the matching method. The mean cup abduction error was 0.85° ± 1.3° (± standard deviation) and the mean version error was 0.01° ± 1.99°. Conventionally measured cup abduction ranged from 44° to 62° and correlated significantly (p = 0.001, r = −0.5) with pelvic flexion angle, proving the linear negative correlation between pelvic flexion and the error in conventional radiographic cup measurements. The Xalign method offered reasonable accuracy for cup orientation, and allowed cup and pelvic 3D anatomic measurements at different times.

Relation Between Vertical Orientation and Stability of Acetabular Component in the Dysplastic Hip Simulated by Nonlinear Three‐dimensional Finite Element Method

In acetabular dysplasia, more vertical orientation of the acetabular component is often used to minimize the superolateral bone grafting. This study was designed to determine the effects of vertical orientation of the cup on the stability and polyethylene wear of the acetabular component in uncemented total hip arthroplasty (THA). Three-dimensional finite element models of the hemipelvis with dysplastic acetabulum were developed. Metal-backed hemispherical cups were placed in the true acetabulum with abduction angles of 35, 45, 55, and 65 degrees. It was found that more vertical orientation of the cup was associated with larger relative motion of the metal shell between the acetabulum and metal shell. Furthermore, tilting and torsional shear stresses in the model of the cup abduction angle of 65 degrees were found to be 1.7 times larger than that in the model with 35 degrees at the bone-metal shell interface. More vertically oriented cups caused larger contact stresses at the articulating surfaces of the polyethylene liners. The results suggest that the abduction angle of the acetabular component significantly influences cup loosening and polyethylene wear in THA.

Acetabular reconstruction of total hip replacement in the treatment of dysplastic hip

To summarize the surgical experience of acetabular reconstruction of total hip replacement in the treatment of dysplastic hip. From April 1998 to April 2002, 96 adult cases with osteoarthritis secondary to dysplastic hip were adopted in the study. Among them, there were 89 females and 7 males, age from 27 approximately 68 years. According to Hartofilakidis classification system, there were dysplasia in 73 hips, low dislocation in 18 hips and high dislocation in 21 hips. All patients received total hip replacement, 16 cases with bilateral replacement and 80 cases with unilateral replacement. Kocher-LangenBeck approach was used during operation and all the acetabular cups were reconstructed at the original anatomic location. The fixation methods were as follows: cement cup for 16 hips, cementless cup for 96 hips and bone graft in 11 hips. The reconstructive methods were as follows: regular replacement for 83 hips; installing a smaller cup after deeper reaming the acetabulum for 27 hips; installing a smaller cup after autofemoral grafting on the superior lip for 2 hips. The incision healed primarily and no infection or nerve injury occurred. Follow-up for 1 approximately 5 years (average for 3.5 years) in 85 patients, the Harris score increased from 33.9 preoperatively to 89.3 postoperatively, and 95 hips had good or excellent clinical results. Radiographically, the positions of the prostheses were normal, the average abduction angle of the cup was 44 degrees, the average superolateral bone coverage of the hips was 96.6%, no radiolucent line was observed at the acetabular side. All the bone grafts fused with the host bone successfully. Total hip replacement is an effective operation in the treatment of osteoarthritis secondary to dysplastic hip. The four specific factors that should be considered in acetabular reconstruction are: (1) the surgical approach, (2) the reconstructive position of the acetabulum, (3) the reconstructive method of the acetabulum, (4) the leg-length discrepancies. It's a challenging to the orthopaedic surgeon. Careful preoperative preparation is recommended.

Reduction in Variability of Acetabular Cup Abduction Using Computer Assisted Surgery: A Prospective and Randomized Study

Optimal orientation of the acetabular component of a total hip prosthesis is an important factor in determining the early and long-term result of a total hip arthroplasty (THA). Conventional positioning of the cup component is usually done using a free-hand method, or with the help of a mechanical acetabular alignment guide. However, these methods have proven to be inaccurate, and a great variation in orientation of the cup is found postoperatively. In this study, we wished to determine if the variability of the abduction angle of acetabular cups could be reduced with the use of computer navigation. The abduction angles of the acetabular components of three groups of 50 THAs were assessed. In the first group, a free-hand method was used to position the cup component. This group was operated in the period before we started using computer navigation for hip surgery. In the second group, CT-based computer navigation was used to plan and help position the cup. The third group consisted of 50 THA cases in which a free-hand method was used to position the cup, although these procedures were performed in the period after we had begun using the Computer Assisted Surgery (CAS) system. The variability in cup abduction angle was assessed in all three groups and compared. There was a significant reduction in variability in the CAS group compared to the first group. There was also a reduction in variability in the CAS group compared to the third group, although this was not statistically significant. It is concluded that the use of computer navigation helped the surgeon to place the cup component with less variability of the abduction angle, and, more importantly, we found that no cups were placed in the more extreme positions (outliers).