Uncemented Porous Tantalum Acetabular Components: Early Follow-Up and Failures in 599 Revision Total Hip Arthroplasties

Abstract

Background

The purpose of this study was to determine the early outcomes of 599 cases of revision THA performed using a porous tantalum cup.

Methods

Clinical and radiographic data was sought in all patients at a minimum two years follow-up, after acetabular revision performed with a porous tantalum cup.

Results

Of the 599 cases identified, there were 51 re-operations in 47 patients (7.8 percent). Cup removal was required in 14 of these cases (2.3 percent). The most common cause for cup removal was a septic joint (12). No cups were revised for aseptic loosening during the study period. There was one case of early cup migration. There were 17 incomplete lucencies not initially seen on post-operative films, but identified later, all were non-progressive on subsequent x-rays.

Conclusions

Early results of porous tantalum acetabular components in the revision setting demonstrate good initial stability and low re-operation rates at two years follow-up.

Level of Evidence

Level 4: Case series

Introduction

Acetabular components for total hip arthroplasty (THA) made from the porous metal tantalum (TM; Zimmer/Implex, Warsaw, IN) have been in use since 1997. Clinical application of this material has grown due to remarkable bone in-growth properties¹³. Additional unique features include high porosity with fully intercommunicating pores, which give it the potential to act as a conduit for local delivery of substrates such as antibiotics, bone growth factors, and bisphosphonates⁴ A roughened surface micro-texture provides a higher coefficient of friction for increased initial stability upon implantation^5-7. Moreover, it has a lower modulus of elasticity than that seen previously with solid metals, which creates the potential for a more physiologic transfer of forces to the pelvis and reduces acetabular stress shielding^7-9. Interestingly, equivalent or lower bacterial adherence to porous tantalum has been demonstrated when compared with traditional surfaces¹⁰.

Successful clinical applications of porous tantalum have been reviewed by a number of authors. Broad clinical applications to orthopaedics were demonstrated by Bobyn et al.¹¹, and more recently those specific to primary and revision THA^12,13 have been examined and summarized. The technique for implantation of a porous tantalum cup and augments, and the associated clinical decision making was nicely reviewed by Drs. Gross, Backstein and colleagues who have extensive clinical experience with these tools¹⁴.

In the present study we sought to determine the outcomes in the first 599 revision THAs performed with a porous tantalum acetabular component at 2 to 8-year follow-up from a single center, examining causes and details related to failures and revision surgery encountered over the study period.

Methods

Institutional Review Board approval was obtained for this study.

Demographic Characteristics

599 trabecular metal cups were inserted during revision acetabular surgery in the period from December 1997 through January 2004. There were 577 patients, 20 of whom had bilateral revisions, and two patients each with two revisions on the same hip. A minimum two-year follow up was potentially available in all patients at the beginning of this study. All cases had been followed prospectively at regular intervals since surgery as part of an Institutional Joint Registry.

The average patient age at surgery was 65.5 years (22 to 94). 345 females and 254 males were enrolled. 276 left and 323 right hips underwent surgery. Mean height was 165.7 centimeters, and weight was 77.3 kilograms, yielding an average BMI of 28.0 (range 14.8 to 54.5). The primary diagnosis at the time of their index surgery was most commonly: osteoarthritis (201), developmental dysplasia (97), proximal femoral fracture (75), unknown (68), rheumatoid arthritis (46), avascular necrosis (37) and previous osteomyelitis or septic arthritis (13).

Surgical Data

Hips had undergone an average of 1.8 previous surgeries (range 1 to 9).

In the 599 revision cases performed, implants included 5 monoblock acetabular components, 1 cup with peripheral screw holes (an early design), and 593 revision shells with multiple screw holes and a cemented liner. The most common indications for acetabular revision were: wear, osteolysis and loosening (387), septic joint (98), failed uni or bipolar hemiarthroplasty (33), and recurrent dislocations (30).

Clinical Follow-up

All patients were followed prospectively, and data was entered into the Institutional Joint Registry. A review of all cases of re-operation, for any reason, was performed.

Radiographic Evaluation

Post-operative images were examined. The presence of radiolucent lines, and the zone in which they occurred was noted, as per the system of Charnley and Delee¹⁵. Cup migration was evaluated using the criteria of Callaghan et al.¹⁶, and the development of any osteolysis was recorded. Minimum two-year follow-up images were reviewed and compared with the post-operative views.

Radiographic reviews were performed by one of three authors [WJL, NON, TAM]. In cases where there was concern regarding loosening defined as: cup migration, new or progressive lucencies, or progressive periarticular osteolysis, a detailed review of all available images, and chart review of clinical notes was performed. These cases were also reviewed by one of the senior authors [DGL].

Results

Clinical

Re-operation and Revisions

51 re-operations were performed. The most common indications for re-operation were instability (24) and septic joint (14) (Tables 1 & 2). Only 14 re-operations involved removal of the trabecular metal acetabular cup (Table 2). The average time to failure following cup insertion was 22.8 months (1 to 69 months). There were 5 males and 9 females, with an average age of 63.3 years (42 to 80 years), and average BMI of 29.1 (24.2 to 41.8). Ten right, and four left hips failed. Of the 14 revision cases where the cup was subsequently removed, the causes for failure were: 12 septic joints, and two malpositioned cups. There were no cases of re-operation for aseptic loosening during the study period.

Table 1.

37 Cases of Revision Surgery with Cup Retention

Diagnosis	Number of Patients
Constrained liner	17
Abductor repair and constrained liner	2
Head and liner exchange	3
Femoral stem revision	4
ORIF peri-prosthetic femur fracture	1
Pelvic discontinuity cage/plate	5
Posterior column plate removal	1
Haematoma evacuation	1
Femoral cable removal	1
I&D with cup retention	2

Abbreviations: ORIF open reduction internal fixation;

I&D incision and debridement

Table 2.

14 Revisions with Cup Removal

Patient	Implant Type	Date of Insertion	Primary Diagnosis	Revision Diagnosis At time of PT cup insertion	Etiology of Cup Failure	Time to Removal (Months)
1	Revision cup	7/31/2000	DDH	Aseptic loosening	Malposition	69
2	Revision cup	10/26/2000	OA	Recurrent instability	Septic joint	57
3	Revision cup	11/8/2000	OA	2nd stage post infection	Septic joint	16
4	Revision cup	2/18/2002	OA	2nd stage post infection	Septic joint	1
5	Revision cup	5/23/2002	OA	2nd stage post infection	Septic joint	21
6	Revision cup	7/16/2002	Post acetabular fracture	Chronic dislocation	Septic joint	22
7	Revision cup	7/16/2002	OA	Aseptic loosening acetabulum	Malposition	1
8	Revision cup	9/19/2002	AVN post steroids	2nd stage post infection	Septic joint	20
9	Revision cup	02/21/2003	OA	Aseptic loosening	Septic joint	39
10	Revision cup	2/27/2003	DDH, L4 myelomeningocele	Aseptic loosening revision acetabular reconstruction	Deep infection at periprosthetic femur fracture	29
11	Revision cup	6/17/2003	OA	Aseptic loosening acetabulum and periprosthetic femur fracture	Septic Joint	12
12	Revision cup	8/28/2003	OA	Aseptic loosening acetabular cage	Septic joint	13
13	Revision cup	9/22/2003	RA	Girdlestone and acetabular fracture post attempted THA	Septic joint	2
14	Revision cup	10/20/2003	OA	2nd stage post infection	Septic joint	17

Abbreviations: AVN avascular necrosis; DDH developmental dysplasia of the hip; OA osteoarthritis; RA rheumatoid arthritis

Radiographic

599 cases were identified for inclusion. There were 44 patient deaths, in only two of these cases was a two-year follow-up x-ray obtained. The cup was cemented into a cage in one case. There were no radiographic signs of cup failure in any of these cases, though in some cases the postoperative radiograph was the only image available. Thirteen of the fourteen cases of cup removal did not have two-year follow-up.

The remaining 543 cases were reviewed. All patients had a minimum postoperative AP and lateral x-ray that was examined. 124 patients did not have a minimum two-year postoperative imaging (20.7 percent), which left 419 cases in which the postoperative x-ray and the most recent follow-up were reviewed and compared. Follow-up was thus obtained to two-years, cup removal, or patient death in 79.6 percent of cases. The average length of radiographic follow-up was 38 months (range 24 to 72 months).

Lucent lines were noted on 83 postoperative x-rays (Table 3). In 37 cases these lines had resolved on the most recent radiograph, and in 4 cases of initial 2 zone lucencies they were reduced but still present. In 26 cases, 18 of which involved isolated zone 3 lucencies, they persisted. The remaining 16 cases did not have complete two-year radiographic follow-up, but no cases of failure or loosening have been reported.

Table 3.

Radiographic review and Radiolucent lines

Zone	Post operative	Follow-up	Incomplete follow-up	New lines
1	6	5 resolved	1	3
2	16	12 resolved; 1 persists	3	0
3	39	14 resolved; 17 persist	8	13
2 zones	22	6 resolved; 4 reduced; 8 persist	4	1
3 zones	0	0	0	1
Totals	83	37 resolved; 4 reduced; 26 persist	16	18

Notes: New 3 zone lucency since removed due to sepsis

Reduced lines only recorded for 2 zone lucencies

1 case of early cup migration occurred but appeared stable at subsequent follow-up

A new lucency was documented in 18 cases. 16 of these were isolated to zone 1 (3) and zone 3 (13). Review of these patients' entire series of radiographs did not reveal any progression of the lucencies.

One case of new zone 2 and 3 lucency was noted. This consisted of a 1–2 mm lucent line with an associated pelvic discontinuity that was treated non-operatively Most recent follow-up imaging, two years following the identification of the pelvic discontinuity, showed healing of the discontinuity and a slight decrease in the thickness of the radiolucent line. The cup was clinically stable, though there was a broken locking ring on the constrained liner.

A single case of early cup migration occurred. Subsequent clinical and radiographic follow-up have not revealed any further migration and the hip was clinically stable and functioning well. The contralateral THA femoral component in this patient is symptomatic and has radiographic signs of loosening.

One case of a complete radiolucent line was noted on our radiographic review. Subsequent analysis of this patient's chart revealed a revision surgery to a constrained liner performed during the early post-operative course for instability. A single culture was obtained and was positive. The patient, initially lost to follow-up, had presented to an outside facility at 39 months postoperatively with a chronically infected THA. Staged revision was performed and cultures grew similar bacteria to the previous operative culture.

Discussion

This study presents the largest series of TM acetabular cups used in the revision setting from a single center. Early clinical and radiographic results are presented at 2 to 8 years post operatively. These cases involved the majority of the most complex acetabular revisions performed over this time course, including 17 cup-cage constructs. Our clinical and radiographic results suggest excellent stability and a low rate of revision in these difficult cases. Though no cases of aseptic loosening occurred in this study group there have been a number of cases that fell outside the timeframe of this study, in the complex acetabular revision setting. This demonstrates that there are limits to the in-growth and stability of these reconstructions as demonstrated by other authors¹⁷.

Trabecular metal cups continue to be used at our center with improved clinical results when compared with previous implant surfaces¹⁸, particularly for the more complex revision acetabular reconstructions. Other authors have noted a similar reduction in revisions with a porous tantalum shell¹⁹, where they also report that the discrepancy was greater in cases with more significant acetabular bone loss.

One early complication reported by Springer et al.²⁰ with the use of a revision porous tantalum cup is the occurrence of an early postoperative transverse pelvic fracture. The average time to diagnosis was eight months, and in seven cases reported, five underwent revision surgery. In all cases solid in-growth of the porous tantalum cup into one of the fracture portions was noted. A cup cage, column plating or both were used to obtain fixation of the fracture with stable early results. It was felt that this complication was the result of cyclical loading on the reduced bone stock following revision reaming and cup placement²⁰. These cases are also included in this current review (Table 1).

A number of clinical reviews have been published. The largest to date is by Skytta et al. who reviewed the results of 827 porous tantalum cups in the Finish national registry. They reported a 3-year survivorship of 92% with an aseptic loosening rate of 2 percent²¹.

Unger and associates reported on the use of mono-block cups for 60 cases of revision acetabular surgery. In five cases the constructs were augmented with peripheral screws. At a mean follow-up of 42 months, 4 cups were revised, one for ‘initial cup movement’, 2 for dislocations, and one for aseptic loosening associated with a pelvic discontinuity non-union. There were no other cases of cup loosening on radiographic review²². At a mean 40-month follow-up Kim et al. noted one revision out of 46 patients with revision shells used for Paprosky 2 and 3 defects²³.

Studies reviewing the combined use of a porous tantalum cup, augments, and in some cases cages, in the complex revision setting with severe bone have demonstrated very low rates of migration and aseptic loosening at early to mid-term follow-up loss^24-34. In a paper by Kosashvili et al. 23 of 26 acetabular reconstructions involving a cage-in-cup technique for pelvic discontinuity were noted to be stable at early follow-up³⁵. They also obtained stable revisions using porous tantalum cups in 12 of 15 cases involving failed acetabular cages³⁶.

Sporer and Paprosky presented 13 cases of porous tantalum cups for treatment of a pelvic discontinuity³⁷. No re-operations were performed, though one case showed possible signs of radiographic loosening.

Concerns associated with our clinical review include the retrospective nature of the review without a comparison group, our follow-up rate, and that clinical outcome scores were not included. Follow-up was incomplete for 20 percent of patients. This occurred despite a rigorous joint registry program with special emphasis on obtaining follow-up in these patients. No re-operations or complications were reported in the patients not reviewed, but it is difficult to obtain patient follow-up and x-rays when they are functioning well without symptoms, or have difficulty getting x-rays approved by payers. This review was intended as an examination of the use of this new porous metal acetabular component, and not as a functional review of the associated revision hip reconstruction and thus clinical outcome scores were not included.

With respect to our radiographic review, it can be difficult to assess the cup interface in complex revision cases involving augments, cages and plates and associated bone grafting. In these cases we reviewed all of the multiple views provided and came to a consensus amongst the reviewers regarding fixation status and presence or absence of lucent lines, though some occult lucencies may persist.

The strengths of this study are the large number of patients, and the fact that all cases were performed at a single center. These results suggest very successful early clinical and radiographic outcomes with the use of a porous tantalum cup for revision acetabular surgery. Specifically this study presents the largest series of porous tantalum acetabular cups from a single center, used in the revision setting. Early clinical and radiographic results show excellent ingrowth and stability of this new surface. The stable apparent bony in-growth seen clinically and radiographically at up to eight years from surgery is encouraging for the likely long-term success of these implants, but further longer-term studies will be necessary to confirm these findings.