Seven additional cases of polyethylene dissociation in pinnacle cup, should we start to worry?

María del Coro Solans-López 1;

Esther Carbó-Laso 1, 2

Rafael Laguna-Aranda 1;

Francisco Chana-Rodríguez 1

Javier Vaquero Martín 1, 2

1- Servicio de Cirugía Ortopédica y Traumatología. Hospital General Universitario Gregorio Marañón, Madrid, España, 2- Departamento de Cirugía. Facultad de Medicina, Universidad Complutense de Madrid. 

Correspondencia:

Dra. M del C Soláns

Hospital General Universitario Gregorio Marañón, Servicio de Ortopedia y Traumatología

Calle Doctor Esquerdo n° 46, 28007, Madrid, España. 

Abstract

Introduction: Polyethylene (PE) liner dissociation is an uncommon but serious complication. The Pinnacle Cup® has shown excellent long-term results, but the number of dissociations reported over the last five years is striking.

Methods: We report seven cases of spontaneous dissociation of the PE liner of the Pinnacle cup® from DePuy (Warsaw, IN, USA) occurring between 2015 and 2022. Six patients had a + 4 10° oblique insert and the remaining one had a +4 neutral insert. Implant orientation in X-rays was reviewed, as well as possible triggering factors and revision surgery.

Results: The mean time to dissociation was 32 months (range, 7-60). Dissociated PE liners showed intraoperative deformation or erosion of the rim and rupture of 3 or 4 of the 6 anti-rotation tabs. Ceramic insert replacement was decided in two cases, and cup replacement was performed in the remaining five, either due to malpositioned acetabular component, or suspected damage to the cup locking system.

Discussion: Stem neck impingement on the PE, malorientation or excess load in the upper area of the cups with high abduction angles, added to the lower resistance of the PE locking system of this cup, appear to be the triggering factors in the reported cases.

The recent increase in the number of reported cases of dissociation of the PE liner in the Pinnacle system poses the question of the true incidence of this serious complication and the importance of reporting it to understand the actual data.

Keywords: Polyethylene dissociation. Pinnacle cup, Spontaneous liner dissociation, Modular acetabular components

Resumen

Introducción: La disociación del polietileno (PE) es una complicación infrecuente pero grave. La copa Pinnacle® ha mostrado excelentes resultados a largo plazo, pero la cantidad de disociaciones reportadas en los últimos cinco años es sorprendente.

Métodos: Presentamos siete casos de disociación espontánea del PE de la copa Pinnacle®, DePuy (Warsaw, IN, EE. UU.) ocurridos entre 2015 y 2022. Seis pacientes tenían un inserto oblicuo de +4 10° y el restante tenía el implante +4 neutro. Se revisó la orientación de los implantes en radiografías, así como los posibles factores desencadenantes y la cirugía de revisión.

Resultados: El tiempo medio de disociación fue de 32 meses (rango, 7-60). Los PE disociados mostraron deformación o erosión intraoperatoria del borde y rotura de 3 o 4 de las 6 lengüetas antirrotación. En dos casos se decidió el reemplazo del implante de cerámica y en los cinco restantes se realizó reemplazo de la copa, ya sea por malposición del componente acetabular o por sospecha de daño en el sistema de bloqueo de la copa.

Conclusión: El pinzamiento del cuello del vástago sobre el PE, la desorientación o el exceso de carga en la zona superior de los cotilos con ángulos de abducción elevados, sumado a la menor resistencia del sistema de bloqueo del PE de este cotilo, parecen ser los factores desencadenantes en lo relatado.

Palabras clave: Disociación de polietileno. Copa Pinnacle®, disociación espontánea del revestimiento, componentes acetabulares modulares

Resumo

Introdução: A separação do polietileno (PE)da cúpula acetabular é uma complicação rara, mas grave. A cúpula Pinnacle® mostrou excelentes resultados a longo prazo, mas o número de dissociações relatadas nos últimos cinco anos é impressionante.

Métodos: Relatamos sete casos de dissociação espontânea da cúpula de PE da cúpula Pinnacle® da DePuy (Warsaw, IN, EUA) ocorridos entre 2015 e 2022. Seis pacientes tiveram inserção oblíqua de +4 10° e o restante teve inserção neutra de +4 do implante. A orientação do implante nas radiografias foi reavaliada, assim como possíveis fatores desencadeantes de cirurgia de revisão.

Resultados: O tempo médio de dissociação foi de 32 meses (intervalo, 7-60). As cúpulas de PE dissociados mostraram deformação intraoperatória ou desgaste da borda e rotura de 3 ou 4 das 6 abas de anti-rotação. A substituição apenas do insert de cerâmica foi decidida em dois casos e a de todo a cúpula acetabular foi realizada nos cinco restantes, quer por mau posicionamento do componente acetabular quer por suspeita de dano no sistema de travamento da cúpula.

Conclusão: O impacto do colo da haste femoral no PE, a má orientação ou o excesso de carga na região superior das cúpulas com altos ângulos de abdução, somado à menor resistência do sistema de travão do PE desta cúpula, parecem ser os fatores desencadeantes nos casos relatados.

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Palavras-chave: Separação do polietileno. Pinnacle cup, Separação espontânea do revestimento, Componentes acetabulares modulares

Introduction:

The modularity of the acetabular component [1] provides greater versatility and allows for a more accurate reproduction of the patient’s hip anatomy but introduces an additional factor of potential failure. Dissociation of the polyethylene (PE) insert is a rare (0.17%- 2,4%) [2][3] but severe complication. Although the Pinnacle cup® from DePuy (Warsaw, IN, USA) has shown excellent results in terms of osseointegration and survival, attention should be drawn to the cases of PE liner dissociation reported in recent years.

The Pinnacle cup incorporates a Morse taper locking mechanism that allows for implanting inserts of different materials (metallic, ceramic and PE). Previous biomechanical studies show lower resistance to push-out and lever-out compared to other designs [4]. Moreover Perkins et al. concluded that the lever-out strength of Pinnacle liners is shown to reduce significantly over time compared to another commonly used acetabular system [5].  

There are five types of PE insert in the Pinnacle system: neutral, +4 neutral (lateralizes 4 mm the rotation centre of the hip), +4 10° (lateralizes 4mm and changes tilt or version 10°), lipped (with an elevated rim that increases coverage) and constrained.

We report seven cases of spontaneous dissociation of lateralized PE inserts occurring between 2015 and 2022 at our centre, and analyse the clinical presentation, X-rays, intraoperative findings, and most likely mechanisms of failure.

Patients and Methods:

Patients and surgical procedure

Five patients were women (71,42%), mean age of the seven patients was 61.43 years (range 35-81), and mean time from total hip replacement to revision for PE dissociation was 32 months (range 7-60). In all patients, a total hip prosthesis with a Pinnacle cup and a Corail or S-ROM stem were implanted. A +4 10° oblique PE insert was used in six patients, while a +4 neutral PE insert was used in the remaining patient (Table 1). A posterolateral approach was used. The raised rim of the oblique PE was positioned posteroinferiorly in four patients, posterior-superiorly in one patient and posterior in the remaining one. Hip stability and range of motion were checked before closure, and no impingement was noted (Table 1.)

Table 1

In the informed consent forms for hip prosthesis placement and replacement, patients authorise the use of their clinical histories, including images, in medical publications.

Component orientation was analysed in CT scans and plain X-rays. The method described by Widmer [6] was used for measuring cup anteversion. Cup tilt was calculated as the angle between a line tangential to the acetabular component and a line joining both tears [7].

Case reports

Case 1

A 33-year-old woman with bilateral osteoarthritis (OA) secondary to developmental dysplasia of the hip, underwent left hip arthroplasty (Pinnacle- S-ROM) without complications. An episode of atraumatic anterior dislocation occurred four days after. Plain X-rays and CT scan showed an abduction angle of the acetabular component of 38°, a slight excess of anteversion 36° and a global offset deficiency. Revision surgery was performed, we reduced the neck stem anteversion and increased offset using a longer head (28+3) and a lateralizad neck. The original +4 10° oblique PE was kept posteroinferiorly.

Twenty-four months after surgery the patient reported sudden right inguinal pain and crunching. X-rays showed an eccentric position of the prosthetic head, and PE dissociation or rupture was suspected. Revision surgery confirmed PE dissociation with wear of the raised rim, suggesting impingement, and rupture of four of the six anti-rotation tabs. The acetabular component was retained, a new ceramic insert and a 28+6 head were inserted.

Case 2

A 66-year-old woman, reported sudden functional inability to walk accompanied by snapping on limb mobilization 43 months after total right hip replacement (Corail, Pinnacle cup 48, ceramic head 32, +4 10° oblique PE oriented posteroinferiorly). Radiographic findings were consistent with PE dissociation with adequate acetabular abduction and anteversion, 41° and 28° respectively, as measured in plain X-rays (Figure 1) (Figure 2). During revision surgery, adequate component orientation was confirmed. The insert (+4 10° oblique) showed signs of wear and deformation in the posteroinferior elevated rim, (Figure 3). The oblique insert did not provide greater stability but caused impingement, so it was exchanged to a ceramic insert and a 32+5 head.

Cases 3 and 4

A woman underwent total hip replacement for bilateral hip OA at 78 and 81 years of age. Atraumatic dissociations of the PE inserts were diagnosed 17 months after right hip replacement and 7 months after left hip replacement respectively. X-rays showed excessive cup tilt and anteversion (right 65°/33° and left 62°/35°). The right insert was a +4 10° oblique PE oriented postero-superiorly, and the left a +4 neutral PE. During surgery, both showed posterosuperior wear and rupture of the insert, and four of the six anti-rotation tabs were broken or deformed. Both cups were replaced, the right cup by a cemented constrained cup, and the left cup by another Pinnacle cup with ceramic insert and head.

Case 5

A 39-year-old man underwent total hip replacement due to bilateral avascular necrosis (AVN) of the femoral head (Corail, Pinnacle cup 48, ceramic head 32, oblique PE oriented posteroinferiorly). 20 months after surgery, at the age of 40, he presented with a crunching sensation, without trauma. X- rays confirmed PE dissociation, acetabular abduction angle was 42° and anteversion was 35°. In revision surgery, we found gross metallosis, PE dislocation with posterosuperior wear and three anti-rotation tabs broken. Cup replacement was performed due to the suspected failure of the fixation mechanism.

Case 6

A 58-year-old man with right hip OA underwent total hip replacement in 2017 (Corail, Pinnacle cup 52, ceramic head 36, +4 10° oblique PE oriented posteriorly). Five years later, at the age of 63, he presented with sudden groin pain without trauma. X- rays revealed and eccentric position of the prosthetic head, abduction angle was 53° and anteversion was 29°. In revision surgery we found the PE to be dislocated, posteriorly deformed and discrete metallosis. Although the components presented an orientation at the safety limits, the hip was not stable when using a neutral PE, so we opted to change the cup and its orientation and use a dual mobility.

Case 7

A 60-year-old-woman underwent total hip replacement (Corail, Pinacle cup 48, ceramic head 32, +4 10° oblique PE oriented posteroinferiorly). Six years later, at the age of 66 she reported sudden pain and squeaking. Radiographs demonstrated subluxation of the head superiorly within the cup, excessive abduction (60°) and anteversion (37°). We decided to perform a cup replacement, the PE showed erosion and fracture of three antirotation locking tabs in its upper rim.

Discussion:

PE dissociation is a very uncommon cause of total hip replacement failure. The true incidence of this complication is unknown. Less than 60 cases have been reported with the Pinnacle system since the first was published in 2009 [8]. The first references to PE dissociation in a modular cup were with the Harris-Galante 1 device (Zimmer, Warsaw, IN, USA), and were attributed to fatigue of the insert locking mechanism [9]. Years later, the same complication, related to possible impingement, was reported with the Harris-Galante 2 cup (Zimmer, Warsaw, IN, USA), the Metasul cup (Zimmer, Warsaw, IN, USA) with its metallic insert, and the tripolar Trident cup (Stryker, Mahwah, NJ, USA).

A total of 582 Pinnacle cups with +4 neutral or +4 10° oblique PE inserts were implanted in our institution between 2015 and 2022. The incidence of PE dislocation at our institution between 2015 and 2022 is 1,2%. Previous authors published rates of 0.17% in 4751 Pinnacle systems [10], 0.83% after analysing 2646 patients [2], and 2.4% of 253 arthroplasties according to Singleton [3]. Some authors suggest that the frequency of this complication may be underestimated because not all cases are reported.

The anchoring system of the Pinnacle cup is a 10° Morse taper adjacent to the cup equator for metal and ceramic inserts. To fix the PE insert, the cup also has 12 recesses or slots in which the six polyethylene tabs are embedded as an anti-rotation mechanism.

After conducting a systematic review of PE dissociations, we identified 51 cases with adequate information about the cups and PE used; Thirty-seven were neutral PE inserts (72.55%), eleven were +4 10° oblique (face-changing) (21.57%), two lipped (3.92%), and one +4 neutral (1.96%). The PE failed and became deformed in four patients (7.84%), and in the remaining patients (92.16%) 3 or 4 anti-rotation tabs were also broken [2][3][8][10-14].

Impingement is one of the proposed dissociation mechanisms in all types of inserts. Repeated impingement of the stem neck on the PE rim in extreme motion positions or in patients with sagittal spinal deformity [15] with properly oriented cups may eventually lead to PE fatigue and deformation, resulting in breakage of the rim and the anti-rotation tabs.

It seems logical to think that the elevated rim of lateralized inserts may increase the risk of impingement. However, this correlation has not been seen in the previous reported cases from other authors, where most of the dissociated PE liners were neutral. Our seven patients had lateralized +4 PE, in our institution we did not have neutral inserts, we have recently introduced them for cup sizes 48 and above. Unlike other current cups, Pinnacle system neutral PE remains elevated some millimetres over the metallic rim, but the consequences have not yet been analysed.

Another possible cause is inadequate PE impaction due to soft tissue interposition or screw prominence. A decrease in PE thickness when large heads are used in small diameter cups (as in our cases 2,4,5,6,7) has also been suggested as a risk factor.

In our seven patients, we think that impingement (between the stem neck and the elevated area of the insert) and cup malposition were the factors triggering dissociation.

In cases 1, 2, 5 and 6 the cup was within “the safe zone” for abduction/anteversion proposed by Lewinnek et al., [16] (Table 1) as measured by CT and AP radiographs. This constitutes a major limitation to our study, as we haven’t evaluated functional combined femoral and acetabular anteversion [17][18]. In revision surgery, a +4 10° oblique trial insert (oriented as the previous one) was tested in order to replicate the possible impingement, visually checking that the femoral neck impacted on the elevated rim of the PE. In cases 1 and 2, we found no gross damage to the cup locking system or significant metallosis, so we decided to change mobile parts and retain the cup. In case number 5, the cup was changed because we found metallosis, suggesting a possible deterioration of the cup locking system. In case number 6 the instability of the hip was the factor that made us decide to replace the cup orientation.

In cases 3, 4 and 7, cup replacement was performed because the primary cup was misoriented. High abduction angles (65°, 62° and 60°) suggest that overloading the upper area of the PE-cup junction promoted progressive PE deformation. It should be noted that time to dissociation was shorter in cases 3 and 4, a fact already mentioned by Napier, who stated that excessively tilted cups lead to earlier dissociation during the first two years after surgery. We assume defects in component orientation but believe that other systems show a much lower rate of PE dissociation, even in the face of poorly oriented cups.

The recent increase in the number of reports of this serious complication with the Pinnacle system raises concern about its true incidence. Moreover, in the absence of a greater number of dissociations reported with other systems, it seems reasonable to infer that this system may be less permissive of surgeons’ technical errors in component orientation. This makes us wonder, if placing an oblique face changing PE reflects a defect in the cup orientation or hip stability. Would it not be preferable to reorient the cup during primary surgery rather than using an oblique PE, especially when using such a demanding system?

Dissociation of PE in the Pinnacle cup is an uncommon complication, but we think that it should be reported due to the widespread use of this system. Adequate cup orientation, avoiding abduction angles greater than 50°, and careful intraoperative verification of neck-insert impingement over wide ranges of motion should be the main factors to be considered to decrease the incidence of this complication. Use of lateralized PE inserts is not associated in the literature to a greater risk of dissociation, despite being the insert used in our six patients.

References

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FIGURES

Figure 1. AP Pelvis view. Radiographic findings consistent with polyethylene dissociation. Eccentric position of the prosthetic head. Case 2

Figure 1

Figure 2. Lateral hip view. Radiographic findings consistent with polyethylene dissociation. Eccentric position of the prosthetic head. Case 2

Figure 2

Figure 3. Polyethylene erosion and deformation. Head wear. Case 2

Figure 3