Thin-Walled Cross-Linked Acetabular Liners Need Not Exhibit Reduced Locking Strength.

Use of larger diameter femoral heads has emerged as a promising strategy to reduce the risk of dislocation after total hip arthroplasty, but thinning the walls of cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) acetabular liners to accommodate these larger heads may compromise the locking mechanism of the liner. The purpose of this study was to test the mechanical integrity of the locking mechanism in cross-linked and re-melted UHMWPE acetabular components with reduced wall thickness. The locking mechanism of cross-linked (100 kGy/re-melted) acetabular liners in sizes 50/28, 50/36, and 52/36 mm of 1 design was evaluated by lever-out tests and torsion tests. Torsion tests were performed at 2 angles to isolate the liner's locking tabs independent of the contribution of its central post. Lever-out testing demonstrated nominally reduced failure strength in 50/36-mm liners (13.3 N · m) compared with 50/28-mm liners (12.3 N · m; P=.0502), whereas the lever-out strength of 52/36-mm liners was 12.2±0.94 N · m. Failure torques were similar between 50/28- and 50/36-mm liners at 45° and 90°, but the failure torque of size 52/36-mm liners was significantly higher at each angle. The use of larger diameter femoral heads does not compromise the locking mechanism of thinned MicroSeal (Signal Medical Corp, Marysville, Michigan) acetabular liners. Use of a cross-linked UHMWPE acetabular liner, with a locking mechanism that is not compromised when the liner is thinned to a thickness of at least 2.86 mm, appears to be a biomechanically sound construct when articulated with large diameter femoral heads.

Reduced UHMWPE wear using magnesia-stabilized zirconia instead of CoCr femoral components in a knee simulator.

Magnesia-stabilized zirconia (Mg-PSZ) is stable and maintains a scratch-resistant surface in hip replacement, but is untested in knees. We assessed whether using Mg-PSZ instead of cobalt-chromium (CoCr) femoral components resulted in less tibial insert wear, and evaluated changes in topography and roughness of the femoral components. Inserts bearing against CoCr or Mg-PSZ were tested using standard (9 Mc) and aggressive (6 Mc) waveforms. Femoral component surface topography and roughness were evaluated before and after testing by optical profilometry. When bearing against Mg-PSZ, UHMWPE wear rate decreased by 73% (standard) and by 59% (aggressive conditions). After 15 Mc, CoCr components featured deep scratches, and roughness increased five-fold, while Mg-PSZ components were unchanged. Mg-PSZ femoral components may be indicated for high-demand patients and those with metal sensitivity.

Vancomycin concentration in synovial fluid: direct injection into the knee vs. intravenous infusion.

The purpose of this study was to measure joint and serum levels of vancomycin following intra-articular (IA) or intravenous (IV) administration, and to compare the concentrations achieved in the joint fluid. IA vancomycin was only used to treat revision total knee arthroplasty (TKA) due to infection, while IV vancomycin was used as a prophylactic agent in primary and revision TKA. Both IA and IV vancomycin achieved therapeutic levels in the synovial fluid of the knee, but IA delivery of vancomycin resulted in peak levels that were many orders of magnitude higher, and also resulted in therapeutic serum levels. The half-life of IA-delivered vancomycin was just over three hours, and trough levels remained therapeutic in the joint and in serum for 24hours after IA injection.

Reinfected revised TKA resolves with an aggressive protocol and antibiotic infusion.

BACKGROUND: Revision of failed two-stage revision TKA for infection is challenging, and amputation often is the only alternative. QUESTIONS/PURPOSES: We asked whether reinfection after two-stage revision for infection could be controlled with an aggressive revision protocol and intraarticular antibiotic infusion. METHODS: We retrospectively reviewed 18 patients (12 women, six men) who underwent revision for failed reimplantation between January 1999 and January 2008. Mean time from revision for infection to rerevision for reinfection was 5 months (range, 1-18 months). All knees were treated with an individualized protocol that included aggressive exposure, extensive débridement, uncemented components, closure with muscle flaps (seven knees) and other plastic surgery procedures (three knees), and direct antibiotic infusion through Hickman catheters for 6 weeks. Ten knees had one-stage revision; five had débridement, cement spacer, and revision surgery 3 to 4 months later; and three had extensive soft tissue reconstruction before revision surgery. The minimum followup was 2.3 years (mean, 6.1 years; range, 2.3-12.0 years). RESULTS: The mean Knee Society scores improved from 33 preoperatively to 76. Seventeen of the 18 had control of infection and achieved durable fixation and a closed wound. One patient had recurrent infection 13 months after one-stage revision, was revised, and remained asymptomatic 28 months postoperatively after redébridement and vancomycin infusion for 6 weeks. In one patient, soft tissue closure was not obtained and the patient required amputation. CONCLUSIONS: Extensile exposure, débridement, and soft tissue flaps for closure combined with uncemented fixation of revision implants and antibiotic infusion into the knee controlled reinfection after revision TKA.

The influence of sterilization method on articular surface damage of retrieved cruciate-retaining tibial inserts.

This observational study was designed to determine the importance of sterilization method and insert thickness as predictors of articular damage of cruciate-retaining polyethylene components used in total knee arthroplasty. Ninety-nine explanted tibial inserts were evaluated for surface damage. Severe damage modes were observed in 36 of 52 of γ-irradiated inserts but none of those sterilized by ethylene oxide. Articular damage significantly correlated to time in vivo but not to insert thickness. Inserts sterilized by ethylene oxide gas in gas-permeable packaging exhibited a significantly lower damage accumulation rate compared with inserts sterilized by γ radiation and stored in air or an inert environment. γ irradiation and storage in argon instead of air reduced the frequency of severe damage such as delamination but not the overall damage rate.

Reduced wear of cross-linked UHMWPE using magnesia-stabilized zirconia femoral heads in a hip simulator.

BACKGROUND: To reduce wear, the ideal bearing surface in joint arthroplasty should be smooth and hydrophilic. Ceramics generally offer better wettability than metals and can be polished to a smoother finish. However, clinical studies have found no reduction in liner wear when using yttria-stabilized zirconia (Y-TZP) instead of cobalt chromium alloy (CoCr) femoral heads. QUESTION/PURPOSES: We (1) determined whether a hard, diamond-like carbon (DLC) coating would enhance the wettability of CoCr and magnesia-stabilized zirconia (Mg-PSZ) femoral heads without increasing roughness, and (2) compared their wear performance. METHODS: In an observational study limited to CoCr and Mg-PSZ heads, we measured roughness and contact angle on as-received and DLC-coated heads. Eight heads then were subjected to 11 million cycles of wear in a hip simulator against cross-linked ultrahigh molecular weight polyethylene (XLPE) liners. RESULTS: Mg-PSZ femoral heads were smoother and more hydrophilic than CoCr heads. Although DLC coatings did not reduce roughness, they reduced the contact angle of CoCr and Mg-PSZ substrates, which may provide enhanced lubrication in vivo. In hip simulator tests, liners bearing against CoCr heads wore at a greater rate compared with Mg-PSZ heads. The DLC coating on Mg-PSZ heads did not reduce wear further. CONCLUSIONS: The wear rate of XLPE versus Mg-PSZ was seven times less than CoCr heads, probably owing to lower roughness and greater wettability of Mg-PSZ heads. CLINICAL RELEVANCE: The use of Mg-PSZ femoral heads should lead to reduced wear in vivo compared with CoCr heads, but the clinical benefit of DLC-coated Mg-PSZ is unclear.

Minimal backside surface changes observed in retrieved acetabular liners.

Modular polyethylene liners offer versatility in total hip arthroplasty, but the locking mechanism may allow micromotion and backside wear. We evaluated the backside surface of 56 retrieved acetabular liners (mean 5.54 years in vivo, range 0.003-13.1 years) to determine whether damage correlated with liner age in vivo, patient factors associated with higher activity, and polyethylene quality. Half of the liners exhibited minimal damage, half exhibited no damage and none exhibited severe damage. Backside damage significantly correlated only to liner age in vivo. Ten of the 28 liners revised for osteolysis exhibited no backside damage, but the osteolytic cysts were peripheral and did not originate from screw holes. The results suggest that modular polyethylene liners in a porous titanium-coated shell with screw holes can be designed such that clinically significant backside wear is minimal.

Methicillin-resistant Staphylococcus aureus in TKA treated with revision and direct intra-articular antibiotic infusion.

BACKGROUND: Resistant organisms are difficult to eradicate in infected total knee arthroplasty. While most surgeons use antibiotic-impregnated cement in these revisions, the delivery of the drug in adequate doses is limited in penetration and duration. Direct infusion is an alternate technique. QUESTIONS/PURPOSES: We asked whether single-stage revision and direct antibiotic infusion for infected TKA would control infection in patients with methicillin-resistant Staphylococcus aureus (MRSA) infections. METHODS: We retrospectively reviewed 18 patients (18 knees) with MRSA with one-stage revision protocol that included débridement, uncemented revision of total knee components, and intraarticular infusion of 500 mg vancomycin via Hickman catheter once or twice daily for 6 weeks; we used no intravenous antibiotics after the first 24 hours. We monitored serum vancomycin levels to maintain levels between 3 and 10 microg/mL. Minimum followup was 27 months (range, 27­75 months). Mean followup was 62 months, (range, 27­96 months). RESULTS: Infection was controlled at last followup in all but one patient with a recurrence of the MRSA. The patient was reoperated at 5 months; a necrotic bone fragment was removed, the knee was débrided and revised, and the antibiotic infusion protocol readministered. The patient remained free of infection 42 months postoperatively. At 2-year followup, the mean Knee Society score was 83. We observed no radiographic evidence of implant migration. CONCLUSIONS: One-stage revision and 6 weeks of intraarticular vancomycin administration controlled infection in MRSA infected TKA with no apparent complications. LEVEL OF EVIDENCE: Level IV, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.

Second-generation locking mechanisms and ethylene oxide sterilization reduce tibial insert backside damage in total knee arthroplasty.

This study evaluated the effects of polyethylene quality and locking mechanism on damage to the nonarticulating (backside) surface of retrieved tibial inserts in total knee arthroplasty. Inserts with peripheral capture (PC) locking mechanisms and ethylene oxide (EtO)-sterilized polyethylene were hypothesized to prevent major backside damage. A total of 156 inserts were sorted by locking mechanism and sterilization method and analyzed by damage scoring methods. Ninety-seven specimens exhibited burnishing. Significant positive linear correlations were observed between damage score and age in vivo for all combinations, but damage occurred at a significantly lower rate for second-generation PC implants with EtO sterilization. Most specimens in this group were undamaged (46/72), with others exhibiting only burnishing. Sex, body mass index, and weight did not influence backside damage.

Not all zirconia femoral heads degrade in vivo.

Degradation of yttria-stabilized zirconia femoral heads in vivo has been linked to increased roughening and even fracture of the femoral head. To determine whether magnesia-stabilized zirconia is better suited to resist degradation, we characterized the monoclinic phase concentration, surface topography, and microhardness of retrieved zirconia femoral heads. From previous work, we expected yttria-stabilized zirconia heads to undergo considerable tetragonal-to-monoclinic phase transformation in vivo, leading to considerably increased roughness and decreased microhardness, whereas magnesia-stabilized zirconia heads would not experience phase transformation and thus would not roughen or exhibit decreased microhardness. We studied seven yttria-stabilized zirconia and 12 magnesia-stabilized zirconia femoral heads. Yttria-stabilized zirconia heads explanted after 5 years exhibited a rough orange peel-like surface under light microscopy and were rougher than magnesia-stabilized zirconia heads (average roughness approximately 20 nm versus 7.5 nm, respectively), likely attributable to increased mono-clinic phase transformation (approximately 37% by weight) caused by low-temperature aging. The microhardness of yttria-stabilized zirconia heads decreased with age, but the relationship was not noteworthy. In contrast, magnesia-stabilized zirconia retrievals showed no change in monoclinic phase concentration, surface roughness, or microhardness with age. The properties of the yttria-stabilized zirconia evaluated in our study deteriorated in vivo, whereas magnesia-stabilized zirconia did not degrade and appears to be a superior biomaterial for bearing in total joint arthroplasty.