Independent assessment and outcomes of 196 short-tapered stems short-term follow-up and review of the literature

Abstract

Background:

Longer term outcome data are now becoming available for short-tapered femoral stems for cementless total hip arthroplasty. The shorter stem has a metaphyseal fit, loading the bone in this area, leading to physiological bone remodelling. It is also bone preserving, as it is 35 mm shorter. It may be easier to insert through a smaller incision and potentially reduce complication rates. We present a retrospective single surgeon case series of 196 patients (>53% follow-up over 5 years). All patients had the cementless ‘Microplasty Taperloc’ (Biomet). Primary outcome measures were femoral component revision rates. Secondary outcome measures included complications, patient-reported functional outcome scores (Oxford hip) and radiographic evidence of loosening.

Methods:

Patients were identified using electronic software. All were routinely followed up and assessed in clinic since implant introduction in 2009. Oxford hip scores were routinely obtained. A surgeon who had not carried out the procedure independently assessed radiographs.

Results:

One hundred ninety-six patients were identified. The revision rate was 0.5% due to an intraoperative peri-prosthetic fracture of the femur identified on post-operative radiograph. The complication rate was 2%, attributable to: subsidence of the prosthesis (one hip), post-operative dislocation (two hips), one of which required acetabular revision. Oxford hip scores increased on average from 21 to 45 (pre- to post-operatively). There were no signs of radiographic loosening.

Conclusion:

The results show that using the short-tapered stem is proving so far to be a reliable and safe alternative to its longer counterpart, with low complication rates in the short term.

Keywords

arthroplasty hip hip replacement microtaperloc

Introduction

In the UK alone, around 33,000 primary cementless total hip arthroplasties (THA) are undertaken per year.¹ THA is widely accepted as one of the most successful orthopaedic procedures performed worldwide. There is a plethora of data with studies showing good reliability of various implant designs. With reference to the tapered stem, one of the many implants that are available, evidence shows the regular tapered implant has good survivorship. Parvizi et al., for example, describe a revision rate of 1% at 10 years.² Mclaughlin and Lee describe a survivorship rate of the femoral component of 87% at 22 years.³ Even with such reliability, technology and industry are driving implants in various directions, including the use of smaller femoral stems. Biomet (Microtaperloc; Biomet, Inc, Warsaw, IN, USA) introduced a short-tapered stem in 2005 and only now are longer term outcome data becoming more readily available.⁴ The shorter stem is theoretically bone preserving, as the stem is 35 mm shorter, and thus gives greater bone stock if revision is required.⁵ Due to the nature of the short stem, it is theoretically easier to insert through a smaller surgical incision than its larger counterpart, aiding minimally invasive surgery.⁶ Use of smaller incision may reduce complication rates if a minimally invasive approach is taken.⁶ One other benefit of the shorter stem is the physiological loading it theoretically produces. This can produce significantly better bone integration and less bone resorption.⁷ Other studies suggest excellent survivorship^8,9 and lower complication rates with this implant.⁶

The ‘regular’ taperloc implant is an uncemented implant produced by Biomet first introduced in 1983. It is made from Titanium alloy, with a 133° shaft–neck angle. Offset options include ‘standard’ and ‘high’. The stem wedged in shape, which is thought to improve rotational stability and as per the name, the stem is tapered (‘3-degree biplanar’) which allows accommodating the change in stiffness from proximal femur to the less stiff distal portion and increases stem to bone stability. The microtaperloc implant is defined as a short-stem implant as it is 35 mm shorter than its counterpart. The longer stem length may therefore add to stability and it may also aid in guiding the implant appropriately into the canal. The microplasty implant has similar properties to the larger implant, with the stem being the only difference. Both implants have a ‘plasma spray porous coating’, which increases the surface area of the implant and allow bony ingrowth.

We present a retrospective single surgeon case series of 197 THAs with the cementless ‘Microplasty Taperloc’. The primary outcome measure was femoral component revision rate. Secondary outcome measures included complications, patient-reported functional outcome scores (Oxford hip) and radiographic evidence of lucency.

Methods

Unless otherwise contraindicated, all patients requiring primary THA received this implant under a single consultant orthopaedic surgeon. Examples of exclusions where the implant was not suitable included revision procedure, and dysplastic or complex primary hip. This short implant was combined with the Biomet Exceed ABT cup system¹⁰ with all patients receiving ceramic on ceramic bearing. All patients were operated with a posterior approach and were routinely followed up at 6 weeks, 12 months, and if uncomplicated, thereafter annually (by extended practice allied healthcare professionals). The patient-recorded outcome measures used in this study were taken from the latest follow-up. Radiographs were taken immediately post-operatively and then annually. Patients were identified using hospital electronic software. We were able to identify all patients whom had been operated on since the introduction of the short-tapered stem in 2009. From this list, it was possible to identify all patients undergoing THA using the microtaperloc implant. Further evaluation of radiographs allowed direct identification of short-tapered stems. A single surgeon, not involved in operating, collected all of the data. All patients had routine follow-up, including pre- and post-operative Oxford hip scores, which were acquired and analysed retrospectively. Latest Oxford score was used in the analysis. Score sheets were given to patients at follow-up appointments; however, if patients did not fill them in, the only option was to give another at next appointment. As patients were filling in scores at different times, using a fixed time point for post-op data analysis was difficult. Electronic note software for all of these patients was scrutinized for complications. Radiographs were evaluated for evidence of revision, subsidence and loosening using Engh et al.’s¹¹ criteria (radiolucency/osteolysis, cortical hypertrophy, subsidence, stability/stress shielding). A single specialist orthopaedic surgeon undertook radiographic evaluation. A Kaplan–Meier statistical analysis of stem and overall survivorship was produced. All patients were contacted at the time of study via telephone in order to check for other complications that may have been reported out of area. Revision surgery was identified via hospital electronic notes, radiographs and telephone discussion with patients.

Approval for the study was obtained through the Hospitals audit and research board.

Results

One hundred ninety-six patients were identified. One hundred five were male and 91 female. Mean age of surgery was 59 years, with a range of 21–78 years. Follow-up was on average 36 months, with a range of 5 to 75 months. No patients were lost to follow-up and all patients had radiographic follow-up in the post-operative period (after discharge).

Fifty-three per cent of the patients had follow-up over 5 years and 76% of patients had follow-up over 4 years.

One patient required femoral revision, making the revision rate 1%. The first sustained a peri-prosthetic fracture during the operation, which was identified on post-operative X-ray. This was revised to a normal length Biomet stem with cerclage wires. Table 1 below shows the Kaplan–Meier stem survivorship for the stem. Table 2 shows the survivorship for the stem and cup. Figure 1 illustrates a graph of the Kaplan Meier survivorship of the microtaperloc.

Table 1.

Kaplan–Meier survivorship of microtaperloc stem.

Year	Survival	N failed	N at risk	95% CI
0.0000	1.000	0	197	1.000, 1.000
1.0000	1.000	0	189	1.000, 1.000
2.0000	1.000	0	158	1.000, 1.000
3.0000	1.000	0	105	1.000, 1.000
4.0000	1.000	0	59	1.000, 1.000
5.0000	0.983	1	34	0.950, 1.000

Table 2.

Kaplan–Meier survivorship of microtaperloc stem and cup.

Year	Survival	N failed	N at risk	95% CI
0.0000	1.000	0	197	1.000, 1.000
1.0000	0.995	1	189	0.985, 1.000
2.0000	0.995	1	158	0.985, 1.000
3.0000	0.995	1	105	0.985, 1.000
4.0000	0.995	1	59	0.985, 1.000
5.0000	0.978	2	34	0.944, 1.000

Figure 1.

Graph to show Kaplan Meier Suvivorship of the microtaperloc stem.

A further three patients had complications, making the overall complications rate 2%. Two patients had post-operative dislocation. One of these had known hypermobility and one sustained dislocation a year later, which was amenable to a closed reduction. The second patient had a dislocation 2 years later, which was also amenable to a closed reduction. The first patient required acetabular revision due to multiple dislocations. The last patient had subsidence of the prosthesis femoral prosthesis; identified 6 months post-operatively (see Figure 2). As of yet, this has not required revision (18 months post-operative). This patient’s Oxford score increased from 21 to 31 (pre- to post-operative).

Figure 2.

Radiograph appearances of one patient suffering from subsidence of the femoral implant. (a) Pre-operative, (b) immediately post-operative and (c) 6 months post-operative.

Only 78 patients had completed pre- and post-operative Oxford hip scores. Mean pre-operative hip score was 21, and the mean post-operative score 45.

Discussion

In recent years, there has been much discussion regarding the use of short-tapered stems in primary hip arthroplasty.^{5,7,12

–18} To our knowledge, only four other authors have presented data on the Biomet Microplasty implant.^6,8,9,19

Barrington and Emerson⁸ provide a cohort study of regular length taperloc implants compared to the microplasty implant. In his microplasty implant group (n = 901), he states survivorship is 99% at 7 years. Eight implants required revision for loosening (n = 4), peri-prosthetic fracture (n = 3) and infection (n = 1). Molli et al. compare a double taperloc stem to the microplasty implant. The study, however, fails to look at any length of follow-up and focuses on perioperative complications associated with the surgery itself. They focus on perioperative fractures, which are stated to be 0.4%, which is lower than the longer length comparator. Lombardi presents results of 640 patients whom underwent THA with the microplasty implant. Follow-up was on average 7.3 months. The revision rate was 0.9% (n = 6), for reason of peri-prosthetic fracture (n = 5), and breach of the posterior cortex (n = 1). Interestingly, they discuss one patient whom suffers from implant subsidence, almost identical to our patient (Figure 2). Neither patient required revision, but this may highlight an issue with the smaller implant. Hossain et al.,¹⁹ the latest author to publish, mainly looked at patient-recorded outcome measures (n = 33), for which follow-up was 42.5 months. They found no significant difference in outcome measures between standard and short-stem groups, however did not comment on complications or revision rates.

Our results appear similar to those of previous studies, with revision rate of 1%. Previous studies, however, present varying follow-up periods. Our patients were followed up with maximum of 6 years and minimum of 5 months. The complication rate seems slightly higher compared to another author – 2%. Molli et al.⁶ state perioperative complications to be 0.4%, however only looks at fractures, and does not discuss follow-up. Barrington and Emerson only discuss revision and does not address complication rates. The complication rate of 2% in our series appears to be lower than that of the longer length taperloc. Mclaughlin and Lee²⁰ present data to state that the complication rate is 4.4% (albeit follow-up on average 16 years). Parvizi et al.² also present outcome data on the long stem, with a non-medical complication rate of 3.1% (follow-up 11 years average). We support the findings from Molli et al.⁶ with our more extensive data in that the complication rate appears to be lower compared to the larger implant. The authors note detailed analysis of loosening, for example, using Roentgen stereophotogrammetry could not be performed in this study, which is a clear limitation.

Literature on other short implants

Albers et al.²¹ use a DePuy Tri-lock stem and reported one stem revision in 4 years (n = 119). This gives a revision rate of 0.8%. Two patients had greater trochanter fractures, two post-operative dislocation (complication rate 3.4%). These data complement ours. Amendola et al.²² used the same stem as Albers and looked at 261 hips with a mean follow-up of 3 years. Two had revisions, one for thigh pain and the other for infection, making revision rate 0.77%, again similar to ours. They did not comment on complications. Panichkul et al.²³ also documented the use of the same implant. Unlike all of the aforementioned authors, they discuss the use of the anterior approach, which they felt led to a higher rate of femoral stem revisions. They reported 5 stem revisions out of 594 stems in a study period of 3 years, although follow-up not clearly described. We feel this is not directly comparable to our data, nor other data described in this article as an anterior approach were used.

Salemyr et al.²⁴ use the DePuy Proxima implant. He compares 51 patients randomized into a ‘short-stem’ group and ‘regular’ (Biomet bi-metric). Aside from detailed Dual-energy X-ray absorptiometry (DEXA) and Radiostereometric Analysis (RSA) analysis, they describe one peri-prosthetic fracture and one deep infection in a 2-year time frame. One patient required revision and they did not comment on the other (n = 26). They further excluded these two patients from their study. If both required revision, this may illustrate a very high revision rate (7.7%), although this sample is very small compared to other studies.

Schnurr et al.²⁵ discuss 1–11-year follow-up in 1634 THAs, which is the largest series of this type. He describes the use of the ‘Metha short stem’. The results are quite heterogenous due to the nature of the implant and options available. Overall, the total number of revisions was 72, being 4.1%. Revision rate for stem alone was 2.2%. As they mention in the article, the implant itself had documented and known issues and required a design change during the study time frame. We feel for this reason the results are not comparable to our own data or others.

Conclusion

In conclusion, the Biomet microtaperloc cementless stem appears to have a similar complication rate as evidenced in the short-term follow-up, with our good results comparable to similar studies. Revision rates are acceptable and comparable to larger stem implants. Complication rates also appear lower. We acknowledge that data so far are still limited, and further analysis over time is required in order to see if these short implants are truly as reliable as their longer stem implants. The limitation in this study is the short-term nature of the follow-up due to the age of the implant. However, the results so far are very promising.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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