Introduction

In elderly patients, the incidence of a second fracture in the contralateral hip within 2 years of a femoral neck fracture (FNF), ranges from 7 to 12%.

Introduction

Metal-on-metal hip resurfacing (MOMHR) is a good surgical indication for young active patients. However, it cannot be used in patients with an excessively short femoral head/neck. To address these cases, a new surgical technique has been developed comprising femoral head augmentation using impacted morcellized bone grafts.

Introduction

Metal-on-polycarbonate urethane (MPU) is a cutting-edge new bearing technology for hip arthroplasty. The acetabular component consists of a 2.7mm-thick polycarbonate-urethane liner inserted into a specially manufactured uncemented titanium shell coated with hydroxyapatite [(HA) Fig. 1]. The liner is pliable and biomechanically mimics human cartilage. In vitro studies have shown minimal wear, fluid film lubrication, physiological load transmission and shock absorption capacity equal to the normal hip. This system includes prosthetic heads of a diameter 12mm less than the socket diameter. The aim of this study was to clinically assess patients treated with this novel technology in a retrospective single centre study.

A retrospective single-center review has been performed to gather clinical data on the use of polycarbonate-urethane (PCU) as an articulating bearing material inside a cobalt-chrome (Co-Cr) press-fit acetabular shell.

As of January 2010, the Co-Cr shell and PCU liner have been implanted into 25 total hip patients which were retrospectively followed. The indications for use were in 24 cases of osteoarthritis, and 1 revision case. No patient was lost to follow-up. The average follow-up time was 17.6 months (range 8-27). The average age of these patients was 67.9 (range 44-84), the sex distribution was 14 female and 11 male patients, of whom 15 were right and 10 left side. 24 patients received a total hip replacement with the metal acetabular system and a cementless femoral stem and 1 patient received the metal acetabular shell coupled to a cemented resurfacing head.

None of the cases has had a dislocation, revision, dislodgement, or infection. At follow-up, the mean Harris hip score was 98 points (80-99). X-rays showed good bone-implant contact without any osteolysis or bone rarefaction.

A detailed review of the clinical data of these patients shows that a PCU liner inserted into a Co-Cr acetabular shell is as safe and effective as other commonly used acetabular shells in other total hip systems currently available. No new or unintended adverse or device-related events were discovered with the clinical use of PCU in a Co-Cr acetabular shell.

Although the number of displaced femoral neck fractures treated with hip arthroplasty is steadily growing, the outcomes are not as good as for other surgical indications. As a result, there is no consensus on the ideal type of arthroplasty for these patients. Unipolar and bipolar arthroplasty have a low dislocation rate but implant longevity and functional results are suboptimal. Total hip arthroplasty (THA) provides better functional outcomes and implant longevity but it is associated with a high incidence of postoperative dislocation. This constitutes a significant limiting factor for a more widespread use of this procedure.

The TriboFit^® Buffer (Active Implants Corporation, Memphis, Tennessee, USA) is a 2.7 mm-thick cup made of polycarbonate-urethane which mimics the mechanical characteristics of human cartilage. It is a pliable, hydrophilic, biocompatible, endotoxin-resistant material and acts as a stress-absorber, transmitting loads to the subchondral bone in a physiological manner. The TriboFit^® Buffer shows excellent tribology, including ideal fluid film lubrication, low friction, high load carrying capabilities and long endurance.

The TriboFit^® Buffer is fixed using flexible mechanical fixation. With a special instrument, a circumferential groove is cut into the patients’ socket. The TriboFit^® Buffer is seated by applying gentle pressure, with its ledge snapping tightly into the groove. The surgical technique is bone sparing as no acetabular bone reaming is required whatsoever. The TriboFit^® Buffer can be coupled with large diameter cobalt-chromium femoral prosthetic heads of the same dimensions as the patients’ femoral head. By restoring the correct hip anatomy and preserving the original size of the femoral head, hip range of motion (ROM) and stability are optimised.

Within a multi-centre study, 224 patients (63 male and 161 female) with femoral neck fractures were treated with the TriboFit^® Buffer, a large diameter head and either cemented (192) or uncemented femoral stems (32). The mean patient age was 83 years (range 65 to 96).

All surgeries were performed using a standard antero-lateral approach.

Rehabilitation was fast and weight-bearing was as tolerated by the patients. There were no major complications, and in particular, no postoperative dislocations were reported.

At a mean follow-up of one year, X-rays showed good implant stability. The mean Harris hip score (HHS) after one month was 58 points and increased to 80 points at one year (p = < 0.05). The ROM was the same as in the intact hip. Only one patient was revised because of nonimplant-related pain. This patient complained of pain in the surgically treated limb which was in actual fact related to spinal stenosis. Analysis of the retrieved implant revealed a loss of thickness in the superior area as well as minimal weight (approximately 2.4%). The backside revealed evidence of macroscopic wear in the area of directional loading from the femoral head to the acetabulum. The bearing surface showed minimal wear (less than 15 mm3), indicating that the primary wear location was on the backside. Retrieved synovial fluid and tissue analysis confirmed that there was no reactivity and no sign of synovitis.

With femoral neck fracture patients, TriboFit^® Buffer arthroplasty is theoretically superior to both hemiarthroplasty and THA as it should involve the same low risk of dislocation and acetabular bone preservation associated with hemiarthroplasty, together with the same good functional results and consistent implant longevity of THA. Other advantages of this technique include reduced bleeding and short surgical times.

The results of this study show that the new TriboFit^® Buffer arthroplasty technology has the potential to revolutionize the surgical treatment of displaced femoral neck fracture.

Although metal-on-metal hip resurfacing (MOMHR) is becoming a well accepted indication for young active patients with hip deformities, it does not come without its disadvantages. Longterm bone atrophy, serum metal ion elevation, metal ion hypersensitivity and the formation of pseudotumours have all been reported in the literature. It is thus clear that there is a need for novel bearing technology.

A potentially revolutionary hip resurfacing system comes in the form of the TriboFit^® Hip System, which comprises a 2.7 mm-thick acetabular buffer made of polycarbonate-urethane, a hydrophilic, biocompatible, endotoxin-resistant material which mimics the fluid film layer naturally present in hip joints. This is a pliable implant whose modulus of elasticity is the same as that of normal human cartilage, thus providing optimum shock absorption. In addition, it induces lubrication, which is of the utmost importance as friction is almost eliminated, resulting in a subsequent decrease in the production of wear particles. Indeed, in vitro studies have shown that metal wear is 7-fold less than with a comparable metal-on-metal implant.

The TriboFit^® Buffer is implanted using flexible mechanical fixation. With a special instrument, a circumferential groove is cut into the patients’ socket. The TriboFit^® Buffer is seated by applying gentle pressure, with its ledge snapping tightly into the groove. The surgical technique is bone sparing as no acetabular bone reaming is required whatsoever. The TriboFit^® Buffer can be coupled with a select number of metal hip resurfacing femoral components.

In our centre, we have used this novel bearing technology to treat patients with both osteoarthritis (two patients) and avascular necrosis (four patients). The mean patient age was 50 years (range 30 to 63). In five patients who had a well preserved socket anatomy, the TriboFit^® Buffer was implanted without reaming the acetabular bone. In one patient with significant osteoarthritic changes of the socket, the TriboFit^® Buffer was inserted into a specially manufactured uncemented metal shell, using the TriboFit^® Buffer as a liner. The socket was reamed according to the standard reaming technique. In two patients a Birmingham hip resurfacing (BHR) femoral component was used and in the other four an ADEPT component was used.

Rehabilitation was fast and uncomplicated. The mean follow-up of these patients was one year. The mean preoperative Harris hip score (HHS) was 62. The mean HHS at one year was 99 (p = < 0.05). X-rays showed good quality bone at the bone-implant interface. No osteolysis, loosening, or bone rarefaction was observed. At follow-up, two patients resumed sporting activities. One patient resumed skiing while the other resumed biking.

Our pilot study shows that TriboFit^® Buffer hip resurfacing arthroplasty is a valid alternative to MOMHR. Compared to the latter, the major advantage includes significantly lower metal wear generation, without any differences in the functional results. This new technology has the potential to expand the use of hip resurfacing to patients with renal malfunction, metal ion allergy/hypersensitivity and to fertile females.

Metal-polycarbonate urethane (MPU) bearing is a cutting-edge new bearing technology for hip reconstruction. It consists of a 3mm-thick pliable acetabular cup which biomechanically mimics human cartilage and can be coupled with large diameter metal heads. In pristine sockets, no acetabular bone reaming is required to insert the cup. No cement is needed and the cup is simply snapped by hand into a groove made with a special tool. In deformed sockets, the standard reaming technique must be used. The cup acts as a liner inserted into an uncemented metal shell.

MPU bearing has been analysed in comparative in vitro studies. Clinical and radiographic results have been recorded at a minimum follow-up of 2 years in 202 femoral neck fracture patients.

Polycarbonate-urethane elasticity is 20MPa (70 times less than UMHWPE, 10,500 times less than CoCr, p < 0.001). The number of particles generated per step is 1000 with MPU, 1,000,000 with ceramic-ceramic and metal-metal (MOM) (p< 0.001). Fluid film thickness is 0.25microns with MPU, 0.02 with MOM (p< 0.001). At a minimum follow-up of 2 years, X-rays showed good implant stability. In sockets where the buffer alone was implanted an improvement of the supraacetabular bone density was observed over time. Mean Harris hip score after 1 month was 58 points, increasing to 80 points at 2 years (p < 0.05). One patient was revised, due to non-implant-related pain.

The in vitro and clinical data support the use of this novel bearing technology which has the potential to revolutionize hip arthroplasty.

Metal-on-metal hip resurfacing (MOMHR) is a good surgical indication for young active patients. However, it cannot be used in patients with severe CDH and in particular a too short head/neck. To address these cases, a new surgical technique consisting of augmentation of the femoral head with impacted morcellized bone grafts has been developed.

32 osteoarthritis patients following severe congenital insufficiency of the femoral head/neck were treated with MOMHR combined with femoral head augmentation. The required amount of augmentation was calculated on preoperative X-rays and confirmed during surgery. Using special instrumentation, bone chips produced while reaming the socket and trimming the head were impacted on the head to achieve the desired reconstruction and lengthening. Finally, the femoral component was cemented.

Mean patient age was 49+ 9 years (18–66). Median head lengthening was 12+ 2 mm. Mean follow up is 4.2 years (3–6). Mean preoperative Harris hip score (HHS) was 58 and at follow-up 95 (p < 0.05). Mean leg lengthening was 2.2 cm (p = 0.001). In all Gruen zones, bone mineral density (BMD) decreased during the first 3 months. At 2 years in zone 1 mean BMD increased to 96.8% (p = 0.009) and in zone 7 to 102.1% (p = 0.05). A correlation was found between valgus positioning of the femoral components and increased BMD (p = 0.005).

This impaction bone grafting technique expands the use of MOMHR to patients with severe congenital hip dysplasia leading to a more anatomical reconstruction with a full recovery of function and BMD.