Radiographic evaluation of linear wear of bipolar hemiarthroplasty devices in vivo

Nobuhiro Kaku; Shouhei Noda; Tomonori Tabata; Hiroaki Tagomori; Hiroshi Tsumura

doi:10.1016/j.jcot.2018.07.016

. 2018 Jul 20;10(4):639–644. doi: 10.1016/j.jcot.2018.07.016

Radiographic evaluation of linear wear of bipolar hemiarthroplasty devices in vivo

Nobuhiro Kaku ^1,^∗, Shouhei Noda ¹, Tomonori Tabata ¹, Hiroaki Tagomori ¹, Hiroshi Tsumura ¹

PMCID: PMC6611971 PMID: 31316231

Abstract

Background

The indications for bipolar hemiarthroplasty (BHA) have been narrowing as those for total hip arthroplasty (THA) have been expanding in cases of femoral neck fracture and initial stages of osteonecrosis of femoral head (ONFH). It is difficult to measure wear in vivo after BHA because the equipment used has a dual bearing system with an inner head inside a metal outer cup. The present study aimed to (1) use a roentgenograph to measure linear wear in vivo after BHA with different acetabular conditions around the bipolar cup, and (2) compare the linear wear of polyethylene between integral bipolar cup (IBC) and Centrax prostheses.

Methods

From among patients who had undergone BHA in our department after 1996, we analyzed 48 joints with osteoarthritis (OA) and without acetabular cartilage, as well as 25 joints with ONFH of stage 3 or less, in which the acetabular cartilage remains. Two types of bipolar cup prostheses were used: the IBC, which was gamma-sterilized in air using 2-Mrad irradiation, and the Centrax, which was gamma-sterilized in nitrogen using 3-Mrad irradiation. To image the inner head in vivo, we used high-pressure X-ray photography; we measured linear wear of the polyethylene with software using Martell Method 1.

Results

Comparing mean annual linear wear between the OA and ONFH groups, using IBC prostheses in both groups, there was a significant difference (0.213 mm vs. 0.096 mm, respectively; P = 0.0177). There was a significant difference between the Centrax and IBC prostheses in OA patients in terms of linear wear (0.04 mm vs. 0.213 mm; P = 0.0181).

Conclusion

The linear wear of polyethylene in BHA implants can be affected by the material used to manufacture the bipolar cup. Such implants should only be used for appropriate indications.

Keywords: Hemiarthroplasty, Hip, Polyethylene wear, Radiography, In vivo

Abbreviations: BHA, bipolar hemiarthroplasty; THA, total hip arthroplasty; IBC, integral bipolar cup; OA, osteoarthritis; ONFH, osteonecrosis of the femoral head

1. Introduction

Currently, polyethylene with improved polymer crosslinking or vitamin E enrichment is used more widely than conventional polyethylene in total hip arthroplasty (THA), leading to good long-term survival rates, even in young patients.¹^,² This has allowed expansion of the indications for THA to include degenerative diseases such as osteoarthritis (OA) and rheumatoid arthritis (RA), as well as stage ≤ 3 osteonecrosis of the femoral head (ONFH) in young patients.³

In the past, bipolar hemiarthroplasty (BHA) was used to treat OA and RA. However, the survival rate after this procedure was lower than that after THA.⁴^,⁵ With regard to ONFH, several recent studies have reported that the clinical results of THA were good, but those of BHA were insufficient.6, 7, 8 These studies often included stage 4 cases, in which the acetabular cartilage is already damaged, and a smaller number of stage 3 cases, in which the acetabular cartilage remains intact. The modern indications for BHA are narrowing, such that BHA is used as a primary treatment for femoral neck fracture only.

Some reports have demonstrated that BHA showed good clinical results in the treatment of ONFH with intact cartilage, even when using implants with old design and materials.⁹^,¹⁰ Recently, polyethylene with polymer crosslinking or vitamin E enrichment has been used to manufacture bipolar cups. Therefore, BHA should confer long-term results that match those of THA. It is relatively easy to evaluate linear wear using a roentgenograph after THA, and many reports have done so in vivo.¹¹^,¹² However, in vivo wear is difficult to measure after BHA, and few reports have done so, because the equipment used has a dual bearing system with an inner head inside a metal outer cup. Therefore, the present study aimed to (1) use a roentgenograph to measure, in vivo, linear wear after BHA with different acetabular conditions around the bipolar cup, and (2) compare the linear wear of polyethylene between integral bipolar cup (IBC) and Centrax prostheses.

2. Methods

2.1. Patients and ethical statements

In this retrospective study, we included 48 joints with OA and without acetabular cartilage, and 25 joints with stage 3 or less ONFH and almost intact acetabular cartilage from among patients who had undergone BHA in our department after 1996. All patients were Japanese. The study was approved by the Ethics Committee of our University and was performed in accordance with the ethical standards of the 1964 Declaration of Helsinki and its later amendments. All patients provided written consent at their final follow-up after the subject of the study and the different X-ray imaging methods had been explained to them.

2.2. Surgical procedures and devices implanted

In both groups, we used a posterolateral approach without trochanteric osteotomy to perform BHA. In the OA group, after extraction of the head, osteophytes were removed from the inferomedial area of the acetabulum; the shallow and steep acetabulum was then excavated in the mediosuperior direction and prepared using reamers to ensure good support and congruity for the bipolar cup. The distance between the medial pelvic wall of the dysplastic acetabulum and the expected position and diameter of the bipolar cup was assessed on the patients' preoperative radiographs. The ideal cup-center-edge angle is about 20°. The thickness of the medial acetabular floor, which should be at least 5 mm, was measured perioperatively using a depth gauge. An osteotomy of the femur was performed proximally on the femoral neck to raise the level of the articulation and thus secure the greater trochanter in a position that allowed adjustment of the leg length discrepancy. The femoral stem was inserted at the base of the native femoral anteversion. However, more than 10° was required to ensure successful installation using this approach.

In the ONFH group, radiographic findings in stage 3 or lower patients showed that the acetabular cartilage remained macroscopically intact during surgery. When the contralateral femoral head was intact, we measured its diameter on a radiograph before surgery. During surgery, the acetabular labrum was preserved, and the ligamentum teres was removed from the acetabular notch. The diameter of the retrieved femoral head was measured and a trial of the bipolar cup was carried out; the size of the bipolar cup was selected based on the best adsorption seen in this trial. The same senior surgeon operated on all patients using the same technique. Specifically, the acetabulum was reamed in all hips in the OA group, but not in any hips in the ONFH group. Rehabilitation with full weight bearing was begun the day after surgery in both groups.

The mean (range) follow-up period after surgery was 11.2 (1.5–20.6) years in the OA group and 13.2 (8.7–19.8) years in the ONFH group. No revisions were needed in either group. Two kinds of bipolar cup were used: (1) the integral bipolar cup (IBC; Smith & Nephew Inc., Memphis, TN, US), which is irradiated using 2 Mrad and integrally formed with an inner head and an outer cup, and (2) the Centrax (Stryker Orthopaedics, Mahwah, NJ, US), which is irradiated using 3 Mrad and contains a polyethylene ring locking mechanism between the inner head and outer cup (Table 1). The former was sterilized using gamma rays in air, the latter in nitrogen gas. The diameter of the inner head of both bipolar cups was 22.225 mm. The mean (range) outer head diameter was 43.6 (38.0–54.0) mm in the OA group and 45.7 (40.0–52.0) mm in the ONFH group. Both the inner head and outer cup were made of a cobalt–chromium alloy.

Table 1.

Production elements of the polyethylene in the two bipolar cups.

Manufacturer	Product name	Material powder	Heat treatment	Processing method	Radiation dose	Ambient conditions at the time of sterilization	Locking methods
Stryker	Centrax bipolar	GUR1050	Annealing	Rod	3 M rad	In nitrogen	Locking link compression,
Smith & Nephew	IBC	GUR1050	No heat treatment	Ram extrusion	2 M rad	In air	integrated type

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Notes: There are process differences, such as ambient conditions at the time of sterilization and radiation dose.

Abbreviations: IBC, integral bipolar cup.

The stem was fixed to all patients without cement. We used 55 modular titanium stems (3M, St. Paul, MN, US), seven Synergy stems (Smith & Nephew Inc., Memphis, TN, US), and 11 Super secure-fit stems (Stryker Orthopaedics, Mahwah, NJ, US).

2.3. Imaging and analytical procedures

To image the inner head in vivo, high-pressure X-ray photography was performed with a voltage of 130 Kv, a current of 250 mA, and an exposure time of 40 m s using a flat panel (Fuji film, Tokyo, Japan) at the patients' last follow-up. The distance between the lens and film was 130 cm. The irradiation was limited to the area around the bipolar cup (Fig. 1, Fig. 2), and the shooting conditions were the recumbent and standing positions. We measured linear wear of the polyethylene using Roman VI.70 software (Institute of Orthopaedics, Oswestry, United Kingdom), applying Martell Method 1 (edge detection) as follows: The inner head or outer cup was indicated and the software automatically drew an approximate outline circle around it, identifying the center of the circle. After the magnification of the X-ray image had been adjusted based on the outer cup diameter, the distance between the inner and outer cup centers was measured, and the total linear wear was calculated as this measured distance divided by the elapsed years. These three measurements were performed by the same examiner: and the average was regarded as the total linear wear amount to subtract the distance between both center points on the original design of the bipolar cup from the distance between both center points at the time of the last follow-up, and the annual linear wear amount was calculated based on the number of years since BHA (Fig. 3). Previous reports have confirmed the reliability of the software used.¹³^,¹⁴

Fig. 1 — A case of osteonecrosis of the femoral head (ONFH)

Joint space can be seen between the contour of the outer cup and acetabular subchondral bone in a patient with ONFH. The contour of the inner head has been clarified with high pressure X-ray photography.

Fig. 2 — A case of osteoarthritis (OA)

There is no joint space and subchondral bone around the bipolar cup in a patient with OA.

Fig. 3 — Measuring linear wear.

Linear wear of polyethylene was measured using the Roman VI.70 software package (Institute of Orthopaedics, Oswestry, United Kingdom) and Martell Method 1 (Edge Detection). Briefly, the outer cup diameter is measured based on the magnification of the X-ray image at the last follow-up. The distance between the inner head center and the outer cup center was then measured. The total linear wear amount was calculated by subtracting the inner head/outer cup center distance on the original drawing from the measurement.

2.4. Radiographic evaluation

We evaluated clinical parameters like leg length, vertical and horizontal center of bipolar cup, lateral acetabular inclination, and femoral stem positioning using the patients' anteroposterior radiographs. Leg length discrepancy was measured as the vertical distance from a straight line connecting the bilateral tear drops of the pelvis to the lesser trochanter, as measured on the anteroposterior radiographs. The center of the bipolar cup was located on the anteroposterior radiographs immediately after surgery and at last follow-up by measuring the vertical and horizontal distance between the center of the teardrop and the center of the bipolar cup.¹⁵ The lateral acetabular inclination was measured as the angle between the transverse diameter line of the bipolar cup opening and the basic line connecting the bilateral teardrops on the anteroposterior radiographs at the last follow-up.¹⁶ Radiographic assessment of linear or focal osteolysis was carried out around the bipolar cup at the same time.⁸^,¹⁷

2.5. Statistical analysis

Welch's t-test and the Tukey–Kramer test, provided in the Statistical Package for Social Science Professional Statistics Package version 22 (IBM Corp., Armonk, NY), were used to compare groups. The level of statistical significance was set at p < 0.05.

3. Results

The mean (range) age, height, weight, and BMI of the patients in the OA group were 58.5 (21.5–88.0) years, 152.3 (138.0–164.2) cm, 56.23 (41.0–78.0) kg, and 24.2 (18.0–34.2) cm/m², respectively; those of the patients in the ONFH group were 52.8 (39.0–65.0) years, 156.9 (141.0–170.0) cm, 52.8 kg (39.0–65.0) kg, and 21.4 (17.3–26.7) cm/m², respectively. The mean (range) preoperative leg length discrepancy was 8.5 (0–21) mm, while the mean intraoperative adjusted distance in length was 9.1 (3–23) mm. In the OA and ONFH groups, the equivalent values were 1.8 (0–5) mm and 1.5 (2–4) mm, respectively. The mean distance between the center of the bipolar cup and the base line connecting the bilateral tear drops, as measured on the anteroposterior radiographs taken immediately after surgery, was 29.7 (15–45) mm horizontally and 32.5 (13–53) mm vertically in the OA group, and 30.9 (17–37) mm and 21.1 (16–32) mm in the ONFH group. In the OA group, the mean cup migration distance was 2.1 (−3 to 6) mm horizontally and 2.8 (0–8) mm vertically at the final survey; the equivalent values in the ONFH group were 1.0 (−2 to 3) mm horizontally and 0.8 (0–2) mm vertically. The mean lateral acetabular inclination of the bipolar cup alignment at the final follow-up was 32.2° (16º–39°) in the OA group and 11.5° (8º–15°) in the ONFH group. Using ≥3° as the criterion for femoral stem misalignment, 43 joints showed neutral alignment, two were in a valgus position and three in a varus position in the OA group. Meanwhile, in the ONFH group, 22 joints showed neutral alignment, one was in a valgus position and two were in a varus position. The OA group had either linear or focal osteolysis around the cup in 64.6% of radiographic evaluations; neither was found in the ONFH group.

The OA group was further divided into two groups, those treated with the IBC and those treated with the Centrax. The ONFH group had only been treated with the IBC. Ultimately, all patients were divided into three groups, the demographic data of which are presented in Table 2. Comparing the ONFH and OA groups using the IBC device, the mean (SD) annual mean linear wear was 0.096 (0.096–0.069) mm in the ONFH group and 0.213 (0.213–0.275) mm in the OA group (P = 0.0177; Fig. 4). In the OA group, there was a significant difference in mean wear between the groups, such that the Centrax device showed 0.040 (0.040–0.037) mm and the IBC device showed 0.213 (0.213–0.275) mm, and the P value was 0.0181 (Fig. 5).

Table 2.

Demographic data of the three groups.

	IBC for OA	Centrax for OA	IBC for ONFH
Number	30 cases, 37 joints	8 cases, 11 joints	19 cases, 25 joints
Sex (male:female)	1:29	3:5	8:11
Age (years)	60.1 ± 9.2	59.1 ± 5.9	50.6 ± 10.8
Height (cm)	151.4 ± 5.3	155.6 ± 5.9	156.9 + 7.7
Weight (kg)	55.4 ± 9.9	59.1 ± 7.6	52.7 ± 8.9
BMI (kg/m²)	24.1 ± 3.9	24.3 ± 2.0	21.4 ± 3.1
Follow-up period (years)	11.9 ± 5.1	8.7 ± 2.6	13.9 ± 3.2
Outer cup diameter (mm)	42.7 ± 4.0	46.6 ± 3.5	45.7 ± 3.8

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Abbreviations: IBC, integral bipolar cup; OA, osteoarthritis; ONFH, osteonecrosis of the femoral head.

Fig. 4 — Comparison of linear wear between osteonecrosis of the femoral head (ONFH) and osteoarthritis (OA) among those using the integral bipolar cup (IBC)

The ONFH group showed significantly less linear wear per year than the OA group.

Fig. 5 — Comparison of linear wear between the Centrax and the integral bipolar cup (IBC) in osteoarthritis

The Centrax group showed significantly less linear wear per year than the IBC group.

4. Discussion

THA is widely used to treat hip OA and RA, as well as stage ≤3 ONFH, in which there is no acetabular cartilage damage.¹^,²^,¹⁸ Furthermore, the indications for THA have been expanded to include femoral neck fracture in the elderly, and the procedure is effective for pain relief, producing stable long-term results in young patients with ONFH and leading to good clinical outcomes.³

BHA has been deemed inferior to THA to treat degenerative diseases such as OA and RA in long-term outcomes.⁴^,⁵ In ONFH, past studies have analyzed all stages, including stage 4 cases that showed poor postoperative results.⁶^,⁷ However, when the indications for BHA were limited to cases with stage ≤3 ONFH, the clinical results were comparable to those of THA, even when an old bipolar cup was used.⁹^,¹⁰ In femoral neck fracture, BHA is not inferior to THA in terms of postoperative hip joint function and activities of daily life.¹⁹^,²⁰ Furthermore, because BHA is less surgically invasive, its postoperative dislocation and revision rates are better than those of THA in patients with femoral neck fracture.21, 22, 23

In cases where the acetabular cartilage is preserved, dual bearing systems that include a self-centering function in the bipolar cup work well in vivo to restore rotational motion to the inner head and outer cup.¹⁶^,²⁴ In this regard, Tsumura et al. compared postoperative migration distances of the bipolar cup between stage 3 ONFH with intact acetabular cartilage and stage 4 ONFH without acetabular cartilage. The mean medial and superior migration distances were, respectively, 0.2 mm inward and 0.8 mm in the stage 3 group and 2.6 mm and 3.7 mm, respectively, in the stage 4 group, indicating that, as migration of the bipolar cup proceeds, further osteolysis occurs around the bipolar cup.²⁵ In other words, the polyethylene wear on the bearing surface did not increase excessively until the migrating bipolar cup had reached the bone.

Many studies have shown that polyethylene liners have high abrasion resistance in THA due to crosslinking. Indeed, even in vivo X-ray images indicate that less wear occurs in the case of crosslinked polyethylene liners than with conventional polyethylene liners.¹¹^,¹² Regarding in vivo polyethylene wear of the bipolar cup, as analyzed by X-ray image measurements, Kusaba and Kuroki published a study investigating the linear wear of the UHR™ (Osteonics, Allendale, NJ, USA), which is an older bipolar cup model.²⁶ In that study, the target disease was osteoarthritis, and the mean linear wear was 0.17 mm over a mean follow-up period of 7.8 years. In the present study, linear wear of the IBC in OA patients was similar to that in Kusaba and Kuroki's study.

The present study also compared OA without acetabular cartilage and ONFH with acetabular cartilage, confirming that there was less polyethylene linear wear on the bearing surface in ONFH than in OA. Because irradiation dose influences the crosslinking of polyethylene molecules and the irradiation environment affects polyethylene oxidation, the polyethylene of Centrax bipolar is probably superior in terms of wear resistance to irradiation dose and environment. Similar to THA, it was also confirmed that the polyethylene wear of the bearing surface is less in an implant produced by a process to prevent the oxidation of polyethylene and generate the crosslinking of polyethylene, such as the Centrax bipolar cup that is gamma-sterilized in nitrogen by irradiation of 3 Mrad.

Because it is difficult to measure the volumetric polyethylene wear of a bipolar cup using radiography, only one report has done so. Specifically, Kobayashi et al. reported the case of a patient with dysplastic OA who had been treated using BHA to ream the acetabulum after bone grafting with screws. A revision was performed 9 years later due to severe bipolar cup migration.²⁷ The same authors computed volumetric wear, using a coordinate-measuring machine (Mitsutoyo BHN 305; Mitsutoyo, Tokyo) to compare the retrieved bipolar cup with a new, unused one of the same size and model. The results showed that the volumetric wear of the bearing surface was 2.3 mm³, whereas that of the polyethylene rim was 159.0 mm³. Thus, the annual volumetric wear rates are 0.26 mm³ and 17.67 mm³, respectively. However, the same report recounted no linear wear results. Conversely, Bose et al. reported a case in which there was osteolysis of the acetabulum and the linear wear rate of the retrieved liner was 0.17 mm/year.²⁸ However, with regard to the relationship between linear and volumetric wear, these two reports cannot simply be compared, because the background and specific bipolar device used differed between them.

The bipolar cup has a dual bearing, and the center of the inner head is located medially to the center of the outer cup. Thus, cup-neck impingement and wear on the polyethylene rim are more likely to occur after BHA than after THA.²⁹

One of the limitations of the present study is that we neglected to measure polyethylene wear from the rim, and we did not compare wear between BHA and THA cases at the same time. Moreover, it was impossible to clarify the effect of creep deformation of the polyethylene bearing surface. Thus, we calculated the average linear wear values from the total penetration. Another limitation is that the demographic data, such as the number of joints in the three groups, were not exactly equal. Since the onset age of ONFH is younger than that of OA, there was a significant difference in age among the IBCs of the ONFH group and other IBC or Centrax prostheses of the OA groups in this study. It is predicted that the wear rate is higher among younger patients who have ONFH with higher activity than among those who have OA. However, the result that the annual linear wear in the ONFH group with younger patients was less than that of the OA group with older patients shows that the remaining cartilage had a higher influence on the acetabulum in the case of polyethylene wear. Recent improvements in polyethylene materials, such as the addition of vitamin E, affect the bearing surface and rim. Furthermore, the rim design has been improved, for instance by flattening its shape. These improvements in the bipolar cup can lead to better long-term results in BHA, reaching a level similar to those recently determined for THA.

In summary, in the present study, the linear wear of the bearing surface after BHA in vivo was lower in patients with ONFH of stage 3 or less, in which the acetabular cartilage remains, than in those with OA, in which there is no acetabular cartilage. Additionally, material improvements led to reduced polyethylene wear, indicating that polyethylene wear on the bearing surface did not increase excessively until the migrating bipolar cup had reached the subchondral bone after BHA to treat ONFH with remaining acetabular cartilage. According to our results, if the indications for BHA are adapted and the materials used improved (e.g., the polyethylene linings), the procedure may demonstrate a long-term survival rate similar to that of THA.

Ethical approval statement

The study was approved by the Ethics Committee of Oita University (Approval No. 943; 30 November 2015) and was performed in accordance with the ethical standards of the 1964 Declaration of Helsinki and its later amendments. All patients provided written consent at their final follow-up after the subject of the study and the different X-ray imaging methods had been explained to them.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Author contributions

N Kaku, S Noda, T Tabata, H Tagomori, and H Tsumura contributed equally to this work; S Noda collected and analyzed the data, and drafted the manuscript; N Kaku provided analytical oversight; H Tsumura designed and supervised the study; T Tabata revised the manuscript for important intellectual content; H Tagomori offered technical and material support; All authors have read and approved the final version to be published.

Declaration of conflicting interests

The authors declare that they have no conflict of interest.

Trial registration number

ISRCTN73824458. Registered 28 September 2014.

Informed consent

Written informed consent was obtained from all individual participants included in the study, or their legal guardian at the last follow-up.

Data sharing statement

Data is available from the corresponding author at nobuhiro@oita-u.ac.jp.

Acknowledgements

We would like to thank Editage (www.editage.com) for English language editing.

Contributor Information

Nobuhiro Kaku, Email: nobuhiro@oita-u.ac.jp.

Shouhei Noda, Email: noda-s@oita-u.ac.jp.

Tomonori Tabata, Email: t.tabata@oita-u.ac.jp.

Hiroaki Tagomori, Email: tagomori@oita-u.ac.jp.

Hiroshi Tsumura, Email: htsumura@oita-u.ac.jp.

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PERMALINK

Radiographic evaluation of linear wear of bipolar hemiarthroplasty devices in vivo

Nobuhiro Kaku

Shouhei Noda

Tomonori Tabata

Hiroaki Tagomori

Hiroshi Tsumura