1 Introduction

Instability (16.5%) was the leading cause for revision total hip arthroplasty (rTHA) within the United Kingdom, according to the recent UK National Joint Registry report.¹ Despite multiple surgical options available, treatment of instability remains a challenge, and the repeat revision rates for instability after rTHA are 14.7%.¹ In revision hip arthroplasty, the removal of a well-fixed uncemented femoral component is demanding and causes morbidity due to excessive blood loss, fractures, and the need for an osteotomy. Some surgeons believe in removing all components and replacing them with new implants as the only way to prevent complications such as secondary dislocations and recurrent revisions, even in elderly American Society of Anaesthesiologists (ASA) Grade III & IV patients. The resultant poor clinical outcomes using this approach have been well documented in the literature.^2,3

Some surgeons have alternatively focused on reducing the adverse effects by using less invasive approaches to resolve hip instability without compromising on hip stability or the patient's quality of life. Revision THA with modular head neck adapters is less invasive than full component revision. It can be performed without a considerable surgical insult and reduces the risks and morbidity related with removal of a well-fixed femoral component. A modular head–neck adapter (Bioball Universal Adapter, Merete Medical, Germany) provides an option to adjust the femoral offset, anteversion, and leg length intraoperatively, alleviating the need for femoral stem revision in selected cases.⁴

Kock et al. suggested that the bioball adapter demonstrated excellent long-term outcomes after revision surgery in patients with multiple comorbidities.⁵ Retrospective multi-centre studies on head-neck adapters suggested that the bioball adapter system restored the femoral offset and version and helped to minimise instability without the need to remove well-fixed components. They also found it to be safe, without any taper junction issues or major complications.^6,7

The Bioball Universal Adapter (BUA) is made of titanium (TiAl6V4) and is available in different lengths (−3 mm to +21 mm; S to 5XL). This enables it to adapt to different morse tapers encountered in practice. The most commonly used tapers are 12/14 and 14/16; tapers 8/10, 10/12, 11/12, 11/13, and V40 are also available. The adapter system is available in a straight and a 7.5-degree offset version, which can be rotated in any plane to restore the version and length (Fig. 1). The adapters must be used only with bioball femoral heads, which are available in both cobalt-chromium and ceramic, with sizes ranging from 28 mm to 58 mm. There is a paucity of data regarding its clinical outcomes in elderly patients with multiple comorbidities.

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Fig. 1 Offset Adapters showing 7.5 degree horizontal offset.

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We present the clinical results following revision arthroplasty with the Bioball head-neck adapter in elderly ASA II, III & IV patients. Our primary aim was to determine the incidence of persistent postoperative instability necessitating further surgery after use of this device. Further surgery was defined as patients requiring closed manipulation of a dislocation or revision surgery for persistent instability after multiple or failed closed techniques. Secondary outcome measures were patient satisfaction rates assessed by the Forgotten Joint Score (FJS) and the Oxford Hip Score (OHS) at a minimum of one year following revision surgery.

The Forgotten Joint Score is a validated patient reported outcome measure (PROM) based on the patient's ability to forget about the prosthetic joint due to successful treatment.⁸ Compared to OHS, FJS is focused on the awareness of the affected joint instead of the pain9. The score ranges from 0 to 100, and a high value indicates that the patient is less aware of the prosthetic joint when performing activities of daily living.^8,9

2 Methods

A retrospective review of our hospital database and the National Joint Registry, UK, was performed after obtaining ethical committee approval. All patients over the age of 55, classed as ASA II, II, & IV, who underwent revision hip arthroplasty using the BioBall adapter over a period of 10 years from January 2010 to January 2020 were included. All the operations were performed by two experienced, high-volume revision arthroplasty surgeons. Six patients were deceased at the time of the latest follow-up and were excluded from the study.

The decision to retain or revise the stem was made using a pre-operative evaluation of radiographs and CT scans. Well-fixed stems with an absence of lysis around the stem and absence of gross malposition of the stem were indications for retention. A plan to use an offset adapter was made if pre-operative assessment showed a reduction in horizontal offset when compared to the contralateral side.

Patient demographic data (age, sex, BMI, and ASA grade) was obtained from the information reported on the NJR revision dataset form (H2) at the time of revision surgery. The surgical approach, indication for revision, intraoperative findings, and components removed or retained were also obtained from the H2 dataset form. The details of the adapter used, and its indication were obtained from the operative notes and collaborated with data submitted on H2 dataset forms. The data was anonymised to protect patient confidentiality.

At surgery, the components were exposed using a posterior or lateral approach. Upon removal of the modular head, the alignment of the femoral stem was evaluated. If the stem was found to be anteverted, a straight adapter was used, and in cases where the stem was in a neutral version, an offset adapter was used to provide anteversion. The offset adapter was also adjusted intraoperatively to provide an increase in length or increase the horizontal offset. If there was gross malalignment of the stem, this was revised during the same surgery. These patients were excluded from our study.

A research nurse contacted all patients to calculate the Forgotten Joint Score (FJS) and Oxford Hip Score (OHS) and inquire about any symptoms of instability or any additional surgery. Follow-up X-rays were reviewed to identify the integrity of the adapter and the components that were retained. The data was collected using MS Excel, and statistical analysis was performed using the data analysis tool in MS Excel (version 2021).

3 Results

A total of 47 patients were identified in whom the bioball adapter was used with retained uncemented femoral stems. There were 31 were females and 16 males. The mean age was 74 years (56–93). 5 patients (10.6%) were classed as ASA Grade II, 19 patients (40.4%) were Grade III, and 23 patients (49%) were Grade IV (Table 1). The mean BMI was 28.5 ± 4.8 SD. The mean follow-up was 52 months ± 28.4 SD.

35 patients underwent revision surgery through a posterior approach and 12 using a lateral approach. The main indications for revision were aseptic loosening of the acetabulum in 21 patients (44.6%), instability in 19 (40.4%), and malalignment of one or more components in 5 patients (10.7%). 2 patients (4.3%) underwent revision due to a periprosthetic fracture of the acetabulum (Fig. 2). The commonest adapter used was medium (20), followed by large,¹³ small,⁴ XL,⁴ XXL,⁴ and 5XL (Table 2).

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Fig. 2 Indications for revision.

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In 18 patients, the adapter was used to provide more lateral offset to improve soft tissue tension and stability (Figs. 3 and 4). In 29 patients, the adapter was used to provide extra anteversion in addition to increasing offset. This was done to compensate for the neutral version of the retained femoral stem noted intraoperatively. 29 metal and 18 Biolox Delta ceramic heads were used. 32 mm was the commonest head size used in 30 cases, followed by 36 mm in 13 cases. In four cases, a 28 mm head was used. The mean drop in haemoglobin noted on the first post-operative day was 2.35 g/dL (range 0.5–4.8). The median FJS was 86 ± 11.6 SD, and the median OHS was 43 ± 6.2 SD at a minimum of 1 year following revision (Table 3).

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Fig. 3 Pre-Operative X-Ray with a malpositioned acetabular component leading to instability.

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Fig. 4 Post Operative X-Ray Showing adapter used to Increase Offset & Neck Length after revision of acetabular component.

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There was no incidence of mechanical dissociation or breakage of the adapter, ceramic head fractures, or infections in any of our cases. One patient who underwent revision surgery for instability developed recurrent dislocation following multilevel lumbar spinal fusion, for which he is awaiting further revision surgery. This was recorded as a failure due to his persistent instability. None of the other patients have experienced instability following revision with BUA. The overall survival rate for the adapters was 98%.

4 Discussion

In revision hip surgery, the quality of life for elderly patients with multiple co-morbidities can be compromised by the undesirable effects of prolonged revision surgery. The most common reason for prolonged surgery is the time taken for the extraction of well-fixed femoral stems. The post-operative mobility following stem extraction is compromised by protected weight-bearing mobilisation for protection of the osteotomy site. This is poorly tolerated by the elderly patients. There is considerable perioperative blood loss following extraction procedures, causing cognitive decline and delayed rehabilitation.¹⁰ To prevent such consequences, a rationalised, less invasive approach in selected cases of revision hip surgery may be validated.^10,11 The use of a modular head-neck adapter is one way of managing this problem, especially when the principal indication for revision is acetabular loosening or instability with a well-fixed uncemented femoral component.^12,13

The leading cause of failure after revision THA is dislocation, with rates ranging from 2% to 16%.¹⁴ Isolated acetabular revisions has the effect of changing the combined ante version of the hip. This is more evident when the revision surgery is performed through a posterior approach and the primary surgery was performed through a lateral approach. Hence, surgeons performing revisions must identify all the factors leading to instability and address them intraoperatively using various options available to decrease the risk of postoperative dislocation. A modular head-neck adapter offers the option of addressing the instability noted intraoperatively due to malalignment between components without changing the well-fixed femoral component.^4–7,12,13 Hoberg et al. investigated the outcome of BUA in a retrospective study with 95 consecutive patients. They analysed serial radiographs and used a Harris Hip Score to evaluate outcomes. They defined failure as repeated revision surgery for any reason. The overall survival rate for the adapter was 92.8%. They did not report any cases of breakage of the adapter or dissociation of the adapter from the stem.¹³ The hypothetical risk of breakage or disassembly of the adapter system has been disapproved in literature.

In our series, we did not observe any cases of mechanical dissociation or failure of the adapter, even when XL/XXL adapters (n = 8) with maximum neck lengths were used. This was also observed by Woelfle et al.,¹² who did not report any incidence of dissociation of the adapter in their series of 18 patients followed up to 6.7 years. They suggested that dissociation or failure of the adapter is highly unlikely based on laboratory tests, and such a case has not yet been reported in the literature. Kretzer et al. conducted an experimental study in 2009 to address concerns about metal ion release and trunnionosis of modular neck adapters. They tested the bioball adapter and concluded that no mechanical failure or extensive corrosion could be identified. They also noted that the titanium ion release measured were extremely low and below the critical range.¹⁵

Recurrent dislocation is the most common cause of re-revision surgery.¹⁴ Woelfle et al.¹² and Hoberg et al.¹³ reported re-revision rates of 15% and 5.2%, respectively. In the study by Dabis et al.,² 6.3% of patients experienced recurrent dislocations, and 1 more patient (3.1%) was symptomatic for instability but had no episode of dislocation. In our series, 1 patient (2.1%) developed repeat instability, which started 3 years after his revision surgery following extensive lumbar spine fusion, resulting in a change in spine-pelvic biomechanics. This cannot be attributed to the failure of the adapter. This compares favourably to the UK national average of 14.7% (NJR UK, 18th Annual Report).

Novoa et al.⁷ published a systematic review of 194 cases from 14 articles that analysed the outcomes of using the BUA in revision hip surgery. They stressed that the main indication for using the BUA is isolated acetabular component revision. They noted that the complications that can be attributed to the implant design were rare and isolated. They also suggested that when BUA is used in the setting of dislocation, the rates of revision surgery are high. This is contrary to our series, where only one patient developed instability following lumbar spine fusion surgery. They concluded that for a bonafide evaluation and validation of this adapter system, more prospective and randomised studies are warranted.

Our experience over the past 10 years has demonstrated that the BUA is associated with a good clinical outcome and very low post-revision instability in elderly patients. The high FJS at a mean follow-up of 4 years following revision suggests that this group of patients have “forgotten” their revised hip. The low incidence of long-term instability along with the avoidance of complications associated with the extraction of well-fixed uncemented stems is encouraging. The BUA is a flexible and reliable system to maximise stability of the hip by modifying the femoral offset, version, and leg length. We would like to stress that the BUA cannot be used to compensate for gross malalignment of the stem, where a complete revision is necessitated. These adapters must be used judiciously after an appropriate preoperative and intraoperative evaluation of hip stability.

We acknowledge that instability in many cases are multifactorial. We feel that in the presence of well aligned components, a BUA is a valuable option to address post operative instability in elderly patients by restoring the biomechanics.

Our study was limited by its retrospective nature and the absence of a control group. The follow-up was short, and we did not determine whether there was a significant improvement in biomechanical parameters such as femoral offset and leg lengths after using the adapter. Improvement in biomechanical parameters may be advocated by the fact that all implants were stable at the latest follow-up and the patients reported no symptoms of instability at a minimum of 1 year following revision surgery.

5 Conclusion

We conclude that the BioBall Universal Adapter System is an excellent option for treating the disabling symptoms of instability in elderly multimorbid patients. It is safe, allows early mobilisation, and leads to reduced morbidity and blood loss in elderly patients who require revision THA. We would recommend its use in these subgroups of patients where the risks of extraction of a well-fixed uncemented stem outweigh the benefits.

Funding/sponsorship

The authors did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Institutional ethical committee approval

Informed consent from patients and institutional ethics board approval were attained. Registration No: L0435.

Author contributions

1. Mr. Mohanrao Garabadi: Writing (original draft & editing), Data Curation, Formal Analysis.

2. Mr. Meraj Akhtar: Data Curation, Resources, Formal Analysis.

3. Mrs. Jody Blow: Data Curation, Resources, Formal Analysis.

4. Mr. Rajesh Pawar: Data Curation, Resources.

5. Mr. Mark Rowsell: Supervision.

6. Mr. Prasad Antapur: Conceptualization, Supervision, and Writing-Review.