How well do lower femoral neck shaft angled stems match patients with coxa vara hip deformities?

Omar Alnachoukati＊, Roger Emerson, Kwame Ennin, Alex Vidal

Texas Center for Joint Replacement, Plano, TX, USA

INTRODUCTION

Reconstructing hip anatomies accurately, with the goal of ultimately optimizing gait, implant function, and natural kinematics, should be the primary objectives of any total hip arthroplasty (THA). Proximal femoral anatomy of a relevant proportion of adult hips are not adequately covered by the most commonly used femoral component.1With the wide variety of patient hip anatomies and subsequent deformities,utilizing standard implants on all patients, otherwise applying a “one size fits all” approach, will not do justice for outlier subset patient populations, particularly those with coxa vara hip deformities (neck shaft angle (NSA) of 120° or less).Because most femoral stems are available in only one or a couple of NSA options, surgeons are often forced to accept a coincidental change in leg length when adjusting the offset.2

Today, a wide range of femoral implants with varying NSA’s are available, and lateralized variants allow for the reconstruction of the femoral offset without influencing leg length.3-5Current implants include a range of NSA options,from 123–135°. In a CT study of 800 adult hips, 5.1% of hips were categorized as coxa vara (NSA ＜ 120°), 69.8%as physiologic (NSA ＞ 120° to ＜ 135°), and 25.1% as coxa valga (NSA ＞ 135°).1Although the prevalence of hips categorized as coxa vara is low in the CT study (Germany),other populations may see a higher prevalence of varus hips, such as American or Asian territories.6Regardless,an alternative approach to THA in these patients should be considered in the pursuit to maximize patient satisfaction.

The goal of the current study is to determine the efficacy of implanting a low NSA femoral stem in patients with coxa vara deformities and determine short-term clinical outcomes, while also utilizing the EOS three-dimensional(3D) imaging system to describe femoral anatomies both pre and postoperatively, analyzing femoral offset, leg length discrepancy, and NSA.

Strengths and limitations

? The ability to focus on pre- and postoperative threedimensional reconstructions of the skeletal system by using biplanar X-rays to obtain beはer clinical outcomes.

? Utilization of the EOS imaging system, a relatively new technology, may obtain accurate and reliable measurements while minimizing radiation exposure.

? A limitation of the study is the small patient cohort used.

? The lack of long-term follow-up of the EOS group.

METHODS

Figure 1: Study design.

The study took place at The Texas Center for Joint Replacement in Plano, Texas, USA. The patient population analyzed in this study were all determined to have coxa vara hip deformities as per preoperative X-ray examination. Given the presence of a short varus neck preoperatively, the senior surgeon decided to proceed with THA utilizing a femoral stem that incorporated a 123° NSA, as opposed to the NSA of 133° found in standard femoral stem implants. The primary goals of restoring offset, equalizing leg length, and matching NSA to the native femur were all measured. All patients implanted with a 123° NSA femoral stem and had completed Harris hip scores (HHS) assessment were included in the clinical outcomes group. All patients implanted with a 123°NSA femoral stem and had completed pre- and postoperative EOS X-ray assessment were included in the EOS group.Both patient cohorts were selected in a consecutive series.

Two patient groups, clinical outcomes group and EOS group, were used for this study. The EOS (EOS imaging, SA, Paris, France) X-ray machine, based on a Nobel prize-winning invention in physics in the field of particle detection, has the ability to capture spatially calibrated anteroposterior and lateral images, providing a 3D surface reconstruction of the skeletal system, which can then be used to analyze rotational conditions of lower limbs and joints in the horizontal plane.7

The clinical outcomes group contained 55 patients who had undergone primary THA surgery from June 2013 to June 2015 by the senior author (RE). The EOS group contained 36 patients who had undergone primary THA from October 2015 to November 2016, by either RE or KE. THA was done through the anterior supine intermuscular approach while utilizing the Taperloc Complete XR 123° femoral stem (Zimmer Biomet; Warsaw, Indiana). All patients who were treated with THA were diagnosed with either osteoarthritis or avascular necrosis. All patients implanted with the Taperloc Complete XR 123° femoral stem were included in the clinical outcomes group. All patients implanted with the Taperloc Complete XR 123° femoral stem and those who completed both pre- and postoperative EOS X-rays were included into the EOS group.

In all cases, the patients were placed supine on the supine hip positioning table with both legs in boots, allowing the surgeon to uniquely position the patient in an effort to replace the hip through a single incision without the detachment of muscle from the femur or pelvis. A C-arm (Ziehm Orlando, Florida, USA) was used to image the diseased hip to determine the native preoperative anteversion, leg lengths, offset, and cup abduction and anteversion. The incision was over the proximal tensor muscle belly, centered over the greater trochanter. The dissection was carried into the interval between the tensor and sartorius muscles. An L-shaped capsulotomy was performed. Retractors were placed around the femoral neck, and the head was resected in a 2-step fashion. On the femoral side, the canal was exposed by placing the leg in extension, external rotation,and adduction using the femoral lift and doing proximal posterior releasing. The canal was prepared with rasps and broaches. After trialing was completed, final components were placed accordingly with restoration of normal leg length in mind.

HHS were obtained both pre- and postoperatively, the most recent clinic visit, in order to assess clinical outcomes following surgery. The HHS, a clinician based outcome measure administered by a qualified healthcare professional, was developed to assess the results of hip surgery with the intention to evaluate various hip disabilities.8,9The HHS score gives a maximum of 100 points, with the higher score correlating with less dysfunction; A total score of ＜70 is considered a poor result; 70–80 is considered fair,80–90 is good, and 90–100 is an excellent result.8,10The EOS was used to retrieve preoperative and postoperative femoral offset, NSA, and femur length (Figure 2). Femoral offset was defined as the distance between the center of the femoral head and the orthogonal projection of this point on the proximal femoral diaphysis axis. Femur length was defined as the distance between the center of the femoral head and the center of the trochlea. NSA was defined as the angle between the femoral neck axis and the proximal diaphysis axis.

Figure 2: Preoperative(left) and postoperative(right) EOS images with neck-shaft angle (NSA),femur length, and offset displayed.

Two-tailed Student’st-tests were used to determine statistical significance, utilizing an alpha of 0.05. Statistical analysis and graphs were prepared using Microsoft Excel(Microsoft Corporation, Redmond, WA, USA) and IBM SPSS statistics 23 (IBM Corporation, Armonk, NY, USA).

RESULTS

Demographics in the clinical outcomes and EOS groups are displayed inTable 1. Mean patient follow-up time was 14 months (1–35 months) and mean HHS follow up time was 12.4 months (5–34 months).

Table 1: Demographics of both groups

Table 2: Pre- and postoperative femoral offset, NSA, and femur length in the EOS group

In the EOS group, the pre- and postoperative values for femoral offset, NSA, and femur length are summarized inTable 2. Mean femoral offset decreased by 3.5 mm, from 41.08 mm preoperatively to 37.6 mm postoperatively. Mean NSA decreased by 0.02°, from 123.02° preoperatively to 123.00° postoperatively. Mean femur length decreased by 2.4 mm, from 404.2 mm preoperatively to 401.8 mm postoperatively. Preoperative NSA for each patient and various femoral stem NSA’s available are displayed inFigure 3.In the clinical outcomes group, mean HHS rose from 47 preoperatively to 93 postoperatively (Figure 4). 80%of patients reported excellent, 10% reported good, and 10% reported fair HHS outcomes at an average of 1 year follow-up. No revisions were reported for either the EOS group or the clinical outcomes group. Hip flexion range of motion (ROM) improved from 87.5° preoperatively to 98°postoperatively (Figure 5).

Figure 3: Scatterplot of 36 patients preoperative NSA’s and various implant stem NSA’s available on the market in the EOS group.

Figure 4: Mean pre- and postoperative HHS scores for patients in the clinical outcomes group at 1-year follow-up.

Figure 5: Pre- vs. postoperative hip flexion range of motion (ROM) in degrees in the clinical outcomes group at 1-year follow-up.

DISCUSSION

Total hip restoration has evolved drastically since its inception in 1960, with the introduction of advanced imaging capabilities and the enhancement in the variety of implants available at a surgeon’s discretion. Optimizing function is contingent on restoring hip mechanics to as near normal as possible, is a generally accepted theme in the THA community.11,12Given the vast array of anatomies in patients, having an equally extensive range of implant options makes customizing THA for individual patients much more feasible. The 123° NSA, compared with other NSA’s available in the market, most closely resembles the mean NSA observed in patients with coxa vara hip deformities.

Coxa vara deformities are a challenging subgroup of THA patients, with an increased likelihood of varus stem positioning, which can contribute to limb lengthening.13Mean preoperative leg length of 404.17 mm was matched to an average of 401.83 mm postoperatively, a match within 2.34 mm. Ranawat et al.14reported that a leg length discrepancy should not exceed 6 mm, and others reported that leg length should be restored within 5 mm.15Beard et al.16found that patients with leg length difference ＞10 mm at follow up of up to 3 years had significantly worse Oxford hip scores. HHS at an average of 12.4 months was reported at 93, indirectly supporting the notion that minimal limb length discrepancy results in greater satisfaction. Although lengthening of the operated limb is more frequent than shortening,14,17,18our series does not support that notion as limb length was actually shortened for the coxa vara patient cohort, by roughly 2 mm on average.

Lower limb length discrepancy may increase the risk of aseptic loosening,19,20gait disorders,21instability,22back pain,23may prompt revision surgery,24,25and is the most common reason for litigation after THA,26stressing the importance of restoring limb length with precision. The absence of revisions reported in either group provides support of correctly restoring limb length during this series.

In order to preserve the same level of abductor muscle tension while continuing to preserve limb length, a lateralized stem must be used when managing coxa vara during THA.12As determined per 3D imaging through the EOS system, the surgeons were successful in matching mean preoperative NSA to mean postoperative NSA within 0.02°, displaying a near perfect match to proximal femoral geometry.

Failure to restore offset is also a reason for patient dissatisfaction.27Average femoral offset decreased from 41.1 mm preoperatively to 37.6 mm postoperatively, resulting in a reduction of 3.5 mm in average femoral offset as measured by the EOS system. Authors conclude that femoral offset discrepancies should not exceed 5 mm, since larger values correlate with increased polyethylene wear.28Sariali et al.29studied the isolated effect of femoral offset after THA on gait and found that a 6 to 12 mm decrease in femoral offset after THA altered gait, lowering swing speed and reducing ROM at the knee when walking. Hip flexion ROM increased from 87.5° preoperatively to 98° postoperatively in the clinical outcomes group. Mahmood et al.30measured isometric abductor muscle strength with an electronic dynamometer in 222 patients and found a reduction in offset of more than 5 mm after THA was associated with a reduction in abductor muscle strength in the operated hip. The ability to remain under the 5 mm offset difference, as compared to patient’s native anatomies, displays further evidence of the practicality of using lower NSA femoral stems in patients with coxa vara hip deformities.

Plotting a linear regression for all patient preoperative NSA’s in the EOS group and also illustrating various femoral implant types from different manufacturers, one can easily visualize how closely the Taperloc XR 123 stem matches the line of best fit for coxa vara patients in this cohort. This figure also helps display the gap between standard implant NSA’s and coxa vara hip patients.

Despite the overall success of THA approximately 7–9%of patients will be dissatisfied with their hip, 1 year after the surgery.31,32No patients reported poor HHS grading in the Clinical Outcomes Group at an average one year following surgery. Although 100% of postoperative HHS grading was fair or better at an average of 1 year post surgery, several studies have failed to demonstrate that any relationship between pre- to postoperative change in NSA, leg length, or femoral offset has any relationship to clinical outcome.11,33,34

Strengths of this study are the ability to focus on preoperative and postoperative imaging, giving the ability to compare similar pre- and postoperative variables (femoral offset, NSA, and limb length). Another strength would be the utilization of the EOS imaging system, a relatively new technology which may constitute an alternative to plain radiographs for obtaining accurate and reliable measurements while minimizing radiation exposure.12,35A weakness of the study would be the small patient cohort used, given the uncommonness of patients with coxa vara deformities.Another weakness of the study would be the lack of long term follow-up of the EOS Group, given its recent adoption in the clinic, and having to study two groups in order to incorporate clinical outcomes and anatomic function into the study.

Although there is a small patient population with coxa vara hip deformities, the variation in patient anatomies requires an array of femoral stems that can help restore patient hip function accurately. The femoral stem examined in this study, the Taperloc Complete XR 123, was proven to accurately restore NSA by 0.03°, femoral offset by 3.47 mm,and leg length by 2 mm, all within clinically documented satisfactory ranges, on average comparing preoperatively to post operation, while reporting no revisions and excellent clinical outcomes. THA surgeons concerned with accurately replicating patient native anatomies in coxa vara deformed hips should consider the use of femoral stems with a lower NSA, as a tool to help restore hip function.

We thank Texas Health Presbyterian Hospital of Plano for their support of this work.

Author contributions

OA was responsible for creation of study design, collection and analysis of data. RE was the senior surgeon. KE was the secondary surgeon. AV was responsible for manuscript editing and data collection and analysis.

Conflicts of interest

The senior surgeon does not receive royalties related to this device,although he is a paid consultant of Zimmer Biomet. Research was funded in part by Zimmer Biomet.

Research ethics

This study received institutional review board approval from the local institution, Texas Health Resources, on April 19, 2017,reference # Pro1617 and was carried out in accordance with Good Clinical Practices (GCP) clinical guidelines.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form, the patients have given their consent for their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Data sharing statement

The datasets analyzed during the current study are available from the corresponding author on reasonable request.

Plagiarism check

Checked twice by iThenticate.

Peer review

Externally peer reviewed.

Open access statement

This is an open access article distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License, which allows others to remix, tweak, and build upon the work non-commercially, as long as the author is credited and the new creations are licensed under the identical terms.

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