Revision total hip replacement (THR) is usually indicated in cases of aseptic loosening, prosthetic dislocation, or infection (1,3). Dorsal hip dislocation is one of the most common complications after THR and has been reported to occur in 4%-15% of early complications after primary THR in large-breed dogs (4,7,10,19). Revision THR is more challenging than primary THR because of anatomical changes in both the femur and acetabulum and alteration of surrounding soft tissues (17). It has been reported that the incidence of dislocation following revision THR is more than three times higher than that of primary THR in humans (14).

Causes of dislocation include severe preoperative hip laxity (luxoid hip) due to poor quality of the joint capsule, low soft tissue tension, periarticular osteophytes with fibrous tissue formation causing impingement, and implant malorientation (10,16). Several surgical methods have been reported to address dorsal hip dislocation, including adjusting the orientation of the cup implant or reimplantation, neck lengthening, and iliofemoral sutures to reinforce hip joint stability (6,13,16). Despite several revision strategies, a standard of care that can lead to successful outcomes has not yet been reported.

The dual mobility (DM) cup system was recently developed to reduce the dislocation of the hip prosthesis, and both successful cases of cemented and cementless DM cups have been reported in veterinary medicine (8,12). The cementless DM cup consists of three components: a smaller head, larger polyether ether ketone (PEEK) hemisphere, and outer acetabular shell (12). In this system, the increase in the range of motion through two articulations between the three components and the increased jump distance according to the large head diameter effectively prevented postoperative hip dislocation.

To date, retrospective studies of clinical outcomes following the DM cup system as a primary THR have been reported in veterinary medicine (8,9,12,18). However, a clinical description of DM cup application in cases of chronic dislocation after primary THR has not been reported. Therefore, we report the surgical description and long-term clinical outcomes of successful management using the DM cup system in cases of unmanageable hip dislocation even after multiple revision surgeries (5).

A 6-year-old, 36.5 kg, castrated male Golden Retriever presented for revision surgery of the left cementless THR, which had been operated 2 years ago. The angles of lateral opening and retroversion were 45° and 20°, respectively, and the neck length was short at the time of the primary THR. The hip dislocation occurred 4 weeks postoperatively and was revised by repositioning the cup orientation and replacing the medium neck with a long neck. However, the patient underwent removal of the cup and head implants due to unmanageable prosthetic hip dislocation occurred 3 weeks after revision surgery. Despite rehabilitation and anti-inflammatory drugs administration, the dog continued to have waxing and waning clinical signs including lameness and reluctant to walk for 22 months.

On physical examination, the dog showed persistent weight-bearing lameness after active exercise of the left hindlimb with mild muscle atrophy. Radiographic examination revealed dorsolateral displacement of the femur with a well-fixed remnant stem and bony proliferation around the cranial and caudal acetabulum rims (Fig. 1). The distance between the center of the neck of the femoral stem and acetabulum was 24.5 mm, indicating severe lateralization of the femoral stem.

Figure 1. Pre- and post-operative radiographs of the left hip joint. (A) Severe laxity of the hip joint before primary THR. Immediate postoperative radiograph of (B) primary THR, (C) revision THR, and (D) at 2 years postoperative radiographs following explantation of the cup implant. (D, E) Dorsolateral displacement of the femur with a well-fixed remnant stem and bony proliferation around the acetabular rims were identified.

The patient was incompatible with the conventional THR revision method, including standard cup reimplantation and a longer neck, due to failure of previous attempts. The surgical plan was to apply the DM cup to increase the free range of motion and jump distance to correct soft tissue elongation and laxity caused by a prolonged period of craniodorsal dislocation of the hip joint.

The dog was premedicated with hydromorphone (0.1 mg/kg intravenously [IV]) and midazolam (0.2 mg/kg, IV). General anesthesia was induced with propofol (6 mg/kg, IV) and maintained with isoflurane in oxygen. Remifentanil was administered by constant rate infusion (0.1-0.3 μg/kg/min) for analgesia. Cefazolin (22 mg/kg, IV) was administered 30 min before the incision and repeated every 90 min. A standard craniolateral approach to the left hip joint was performed.

Revision surgery using DM cup system was performed according to previous surgical description (12). Briefly, preparation of the acetabulum was performed with a series of 26-, 28-, and 29-mm spherical cup reamers. The outer shell of a 29.5 mm DM cup (Kyon Inc., Zürich, Switzerland) was impacted into the prepared acetabulum and fixed with five 2.4-mm cortical screws (Fig. 2). Subsequently, the inner shell was impacted into the outer shell with a coaxial alignment by the spike of the inner shell. The 19-mm ceramic head and carbon fiber-reinforced PEEK head were assembled and impacted on the medium neck. The preassembled DM head and neck were placed on the cup first, and the hip joint was reduced between the neck and peg of the femoral stem. During reduction, a narrow Hohmann retractor was inserted caudally to the acetabulum to lever the femur distolaterally, and the neck was manipulated using straight hemostats to combine with the peg of the femoral stem. The iliofemoral suture was applied using an anchor screw and two PDS sutures to reinforce hip joint stability. The surgical site was copiously lavaged, and a surgical site swab was taken for submission of the culture test. Closure of the surgical site was performed in a routine manner.

Figure 2. (A) Intraoperative photograph of cup implantation. The outer shell of a 29.5-mm DM cup was impacted into the prepared acetabulum. (B) The outer shell was fixed with five 2.4-mm cortical screws. (C) Preassembled head and neck components. (D) Routine closure was performed after reduction of the hip prosthesis.

Radiographs were obtained immediately after surgery, which revealed the intended implantation of the DM cup (Fig. 3). The angles of the lateral opening and retroversion of the DM cup were 38° and 18°, respectively. Amoxicillin-clavulanic acid (12.5 mg/kg, IV) and clindamycin (8.5 mg/kg, IV) were administered postoperatively for two weeks without a positive culture test. Remifentanil (0.1 μg/kg/min CRI) was administered during the first 6 h and then converted to meloxicam (0.2 mg/kg subcutaneously once daily on day 1, 0.1 mg/kg orally once daily on Days 2-7) for seven days postoperatively. The patient was able to bear weight at a controlled leash walk the day after the surgery, and the limb function improved progressively. The patient showed almost normal limb function and was discharged two weeks postoperatively. A stable DM cup and osseointegration were noted on radiographic images at 6 months follow-up (Fig. 3). The owner reported excellent clinical outcomes without hip dislocation until 18 months of follow-up via phone call.

Figure 3. Postoperative ventro-dorsal and lateral radiographs of the left hip joint with DM cup (A-C) immediately postoperatively and (D-F) at 6 months postoperatively. No evidence of loosening or change in implant position and osseointegration between the cup and acetabulum can be observed on the radiographs.

This study described acetabular revision surgery using a dual-mobility cup system in a dog with habitual prosthetic hip dislocation following THR surgery. To the best of our knowledge, this is the first report on the application of a DM cup system to address chronic prosthetic hip dislocation for over 2 years. The clinical outcomes showed excellent limb function without any complications until 18 months of follow-up.

Poor quality of the joint capsule, severe laxity, and contracture of the soft tissues around the hip joint have been reported as risk factors for THR dislocation (10,16). Improper implant selection and orientation have been reported as surgeon-related risk factors for prosthetic dislocation (6). In our case, prosthetic dislocation occurred one month after surgery, even though the cup and stem were implanted according to the anatomic reference of the patient. Prosthetic dislocation could not be addressed despite the repositioning of the cup implant and neck lengthening. Lanz et al. (12) reported that very unstable hip joints called ‘luxoid hips’ in large and giant breed dogs are the risk factors for dislocation with a high incidence of 8.5% within 2 months following primary THR. The patient had chronic hip luxation at a young age and had undergone primary THR at 4 years of age. Thus, poor quality of the soft tissue and severe laxity were considered causes of repeated hip dislocation.

A DM cup system has been developed to prevent hip dislocation and improve prosthetic stability through various mechanisms (9,12). First, the large head diameter of the DM cup increases the femoral head-to-neck ratio, which increases prosthetic stability by increasing the ‘jump distance’ required for the dislocation of the femoral head from the acetabular component (2,11,15). Additionally, the Zurich cementless DM cup has a range of motion of 145°, which is increased by 10° compared to the standard cup through double bearing, and can reduce impingement, one of the major causes of prosthetic dislocation (12). The DM cup system has been successfully applied in both human and veterinary medicine, and our case of chronic hip dislocation could be successfully revised using the DM cup (11,12,17).

According to previous retrospective studies where the DM cup system was applied to the primary THR, dislocation was reported in only one case out of 103 cases in which cementless was applied, and no dislocation occurred in 50 cases in which the cement DM cup was applied (8,12). These clinical outcomes were consistent with the 1%-2% incidence of dislocation following DM cup application reported in human medicine (11,17). In our case, there was no dislocation at the follow-up of 18 months, and an excellent clinical outcome was observed.

The limitations of this case report include the fact that it was based on a single case and there is a lack of extended long-term follow-up of more than several years to observe complications such as wearing and aseptic loosening, even though it showed excellent clinical outcomes in our follow-up periods.

This case report describes a revision strategy for THR using a dual-mobility cup system in a dog with unmanageable hip dislocation following primary THR. Based on the clinical outcomes, dual-mobility cup systems can be an alternative to standard THR systems in dogs with challenges in addressing hip dislocations following primary THR surgery.

This work was supported by Korea Institute of Planning and Evaluation for Technology in Food, Agriculture and Forestry (IPET) through Companion Animal Whole Cycle Industialization Technology Development Program, funded by Ministry of Agriculture, Food and Rural Affairs (MAFRA) (322090).

The authors have no conflicting interests.

Author 1: conception of the study, data collection, interpretation of the results, manuscript preparation, critical review of the article, and approval of the final manuscript; Author 2: conception of the study, data collection, performed surgical procedures, interpretation of the results, manuscript preparation, critical review of the article, and approval of the final manuscript.