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Orthopaedic Proceedings
Vol. 95-B, Issue SUPP_34 | Pages 76 - 76
1 Dec 2013
Hoenecke H Flores-Hernandez C Hermida J Kersten A D'Lima D
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Introduction:

Total shoulder arthroplasty (TSA) is the current standard treatment for severe osteoarthritis of the glenohumeral joint [1]. Often, severe arthritis is associated with abnormal glenoid version or excessive posterior wear [2]. Reaming to correct more than 15° of retroversion back to neutral is not ideal as it may remove an excessive amount of the outer cortical support and medialize the glenoid component [3]. Two recent glenoid components with posterior augments—wedged and stepped—have been designed to address excessive posterior wear and to allow glenoid component neutralization. Hypothetically, these augmented glenoid designs lessen the complications associated with using a standard glenoid component in cases of shoulder osteoarthritis with excessive posterior wear. We set out to determine which implant type (standard, stepped, or wedged) corrects retroversion while removing the least amount of bone in glenoids with posterior erosion.

Methods:

Serial shoulder CT scans were obtained from 121 patients before total shoulder arthroplasty. These were then classified using the Walch Classification. We produced 3D models of the scapula from CT scans for 10 subjects that were classified as B2 using the software MIMICS (Materialise, Belgium). Each of these 10 glenoid subjects were then virtually implanted with standard, stepped, and wedged glenoid components (Fig 1). The volume of surgical bone removed and maximum reaming depth were calculated for each design and for each subject. In addition, the area of the backside of the glenoid in contact with cancellous versus cortical bone was calculated for each glenoid design and for each subject (Fig 2). ANOVA testing was performed.


Orthopaedic Proceedings
Vol. 95-B, Issue SUPP_34 | Pages 277 - 277
1 Dec 2013
D'Lima D Netter J Steklov N Hermida J Chen P Nevelos J
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Introduction:

Microseparation has resulted in more than ten-fold increase in ceramic-on-ceramic and metal-on-metal bearing wear, and even fracture in a zirconia head [1–4]. However, despite the greater microseparation reported clinically for metal-on-polyethylene wear, less is known about its potential detrimental effects for this bearing couple. This study was therefore designed to simulate the effects of micromotion using finite element analysis and to validate computational predictions with experimental wear testing.

Methods:

Experimental wear rates for low and highly crosslinked polyethylene hip liners were obtained from a previously reported conventional hip wear simulator study [5]. A finite element model of the wear simulation for this design was constructed to replicate experimental conditions and to compute the wear coefficients that matched the experimental wear rates. We have previous described out this method of validation for knee wear simulation studies [6,7]. This wear coefficient was used to predict wear in a Dual-Mobility hip component (Fig 1).

Dual mobility total hip arthroplasty components, Restoration ADM (Fig 1), with highly crosslinked acetabular liners were experimentally tested: the control group was subjected to wear testing using the ISO 14242-1 waveform on a hip wear simulator. The microseparation group was subjected to a nominal 0.8 mm lateral microseparation during the swing phase by engaging lateral force springs and reducing the swing phase vertical force.


Orthopaedic Proceedings
Vol. 95-B, Issue SUPP_15 | Pages 56 - 56
1 Mar 2013
Netter J Hermida J Kester M D'Alessio J Steklov N Flores-Hernandez C Colwell C Lima DD
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INTRODUCTION

Wear and polyethylene damage have been implicated in up to 22% of revision surgeries after unicompartmental knee replacement. Two major design rationales to reduce this rate involve either geometry and/or material strategies. Geometric options involve highly congruent mobile bearings with large contact areas; or moderately conforming fixed bearings to prevent bearing dislocation and reduce back-side wear, while material changes involve use of highly crosslinked polyethylene. This study was designed to determine if a highly crosslinked fixed-bearing design would increase wear resistance.

METHODS

Gravimetric wear rates were measured for two unicompartmental implant designs: Oxford unicompartmental (Biomet) and Triathlon X3 PKR (Stryker) on a knee wear simulator (AMTI) using the ISO-recommended standard. The Oxford design had a highly conforming mobile bearing of compression molded Polyethylene (Arcom). The Triathlon PKR had a moderately conforming fixed bearing of sequentially crosslinked Polyethylene (X3).

A finite element model of the AMTI wear simulation was constructed to replicate experimental conditions and to compute wear. This approach was validated using experimental results from previous studies.

The wear coefficient obtained previously for radiation-sterilized low crosslinked polyethylene was used to predict wear in Oxford components. The wear coefficient obtained for highly crosslinked polyethylene was used to predict wear in Triathlon X3 PKR components. To study the effect design and polyethylene crosslinking, wear rates were computed for each design using both wear coefficients.


Orthopaedic Proceedings
Vol. 90-B, Issue SUPP_I | Pages 162 - 162
1 Mar 2008
D’Lima D Hermida J Colwell C
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Highly cross linked polyethylenes have been shown to be substantially wear resistant. Typically, crosslinking is achieved by radiation in a low oxygen environment. While the early wear-simulation data is encouraging, concerns remain about the potential for aging and oxidative damage on exposure to oxygen during storage or in the body. This study measured wear rates in highly crosslinked liners that had been exposed to room air for up to 4 years.

Polyethylene liners were divided into four groups: two groups of highly crosslinked liners, XL (freshly opened) and XL-Aged (aged); and two groups of nominally crosslinked liners, N (freshly opened) and N-Aged (aged). The highly crosslinked liners were crosslinked with 9.5 Mrad of warm electron-beam irradiation, treated to a post-cross linking heat treatment to quench free radicals (WIAM), followed by ethylene oxide sterilization. The nominally cross linked liners were sterilized with 2.5 Mrad. The aged liners (XL-Aged and N-Aged) were stored in saline (at 37°C) exposed to room air for 4 years. Three liners from each group were tested in a hip-wear simulator (90% bovine serum) for 5 million cycles. Gravimetric wear measurements were made at 500,000 cycle intervals.

The N and N-Aged groups wore at rates of 14.76 ±3.1 and 15.58 ±1.21 mg/million cycles, respectively. The wear in both XL and XL-Aged groups was not measurable, resulting in weight gains of 2.73±0.5 and 2.17 ±1.1 mg/million cycles, respectively.

WIAM cross linked polyethylene has been reported to generate the least free radicals and has the least potential for oxidative damage. There have been concerns regarding the validity of artificial aging by the high-temperature oxidation. Aging in saline at body temperature while exposed to room air is more representative of in vivo aging. This data supports the results of artificial aging and the long-term durability of WIAM polyethylene.


Orthopaedic Proceedings
Vol. 87-B, Issue SUPP_III | Pages 350 - 350
1 Sep 2005
Colwell C Hermida J Bergula A Chen P D’Lima D
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Introduction and Aims: Studies have shown substantial reduction in wear rates in elevated cross-linked polyethylene (crosslinking to a higher degree than that obtained by radiation sterilisation alone). The aim of this study was to test the effect of increased crosslinking and increased head size on polyethylene wear rates.

Method: Four groups of acetabular liners from a single manufacturer were tested: 28mm nominally cross-linked, 32mm nominally cross-linked, 28mm elevated cross-linked, and 32mm elevated cross-linked. Three implants from each group were tested in a 12-station hip wear simulator using 90% bovine serum as lubricant. Liners were articulated with the appropriately sized cobalt-chrome femoral head. Additional liners from each design were subjected only to the same load without motion to serve as load-soak controls to account for any weight gain due to fluid absorption. Gravimetric analysis was performed every 500,000 cycles for a total of 5,000,000 cycles.

Results: Nominally cross-linked liners demonstrated mean wear rates of 14.97±2.70 and 16.92±2.58 milligrams/million cycles for 28mm and 32mm head sizes, respectively. Both of the elevated cross-linked liners had significantly lower wear rates than controls with a mean of 1.51±1.08 and 2.57±1.78 milligrams/million cycles for 28mm and 32mm head sizes, respectively (p< 0.001).

Conclusion: Larger femoral head sizes reduce dislocation in total hip arthroplasty; however, they have been associated with unacceptably high wear rates. The dramatic reduction in wear rates with polyethylene crosslinking even with the larger head size may increase the potential for use of 32mm head components in total hip arthroplasty.


Orthopaedic Proceedings
Vol. 87-B, Issue SUPP_III | Pages 301 - 301
1 Sep 2005
Colwell C Hermida J Patil S D’Lima D Dimaano F Hawkins M
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Introduction and Aims: Titanium foam implants simulate the trabecular structure of bone to maximise porous space for bone ingrowth. Plasma-sprayed hydroxyapatite coatings work well on non-porous substrates but do not coat the inner surfaces of open-porous substrates. Chemical deposition is an attractive alternative that produces consistent coats on porous surfaces.

Method: Titanium foam cylinders (5mm diameter by 25mm length) were implanted bilaterally in 40 rabbit femurs. Twenty implants were coated with 20 microns of hydroxyapatite (T-HA) by electrochemical deposition while 20 implants had no hydroxyapatite coat (T). Osseointegration was measured at six and 12 weeks by automated computerised histomorphometry of scanning electron microscopy images of sections taken through the implant at two levels: diaphyseal and metaphyseal. Bone ingrowth was quantified in the pores and was also measured up to 1mm beyond the surface of the implant to determine the pattern of bone growth.

Results: For the T-HA surface, bone ingrowth increased from 35.0 ±8.5 % at six weeks to 41.5 ± 7.4 % at 12 weeks (p < 0.05). For the T surface, bone growth was 14.1 ± 8.8% at six weeks and 11.4 ± 4.2 % at 12 weeks. At both time points mean bone ingrowth was significantly different between hydroxyapatite-coated and non-hydroxyapatite-coated implants, (p< 0.01). No significant differences were noted between the diaphyseal and metaphyseal bone response.

Conclusion: For the T-HA surface, bone ingrowth increased from 35.0 ±8.5 % at six weeks to 41.5 ± 7.4 % at 12 weeks (p < 0.05). For the T surface, bone growth was 14.1 ± 8.8% at six weeks and 11.4 ± 4.2 % at 12 weeks. At both time points mean bone ingrowth was significantly different between hydroxyapatite-coated and non-hydroxyapatite-coated implants, (p< 0.01). No significant differences were noted between the diaphyseal and metaphyseal bone response.


Orthopaedic Proceedings
Vol. 86-B, Issue SUPP_IV | Pages 414 - 414
1 Apr 2004
D’Lima D Hermida J Bergula A Chen P Colwell C
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Polyethylene wear is a significant factor limiting survivorship of total knee arthroplasty (TKR). Crosslinking of polyethylene has been shown to significantly reduce wear in hip arthroplasty but has not been reported for TKR. This study measured wear in polyethylene cross-linked to two levels in a knee wear simulator.

Six polyethylene knee inserts were tested in a knee wear simulator. Inserts were manufactured from polyethylene crosslinked to two different levels: 2.5 Mrad (Low-X) and 10.5 Mrad (High-X). Each implant was enclosed in a closed lubricant (50% alpha fraction calf serum) recirculation chamber, maintained at 37°C and changed every 500,000 cycles. Physiologic levels of load and motion were applied at 1 Hz for a total of 6,000,000 cycles. Wear was measured by the gravimetric method before wear testing and at every 500,000 cycles. Semi-quantitative wear assessment was performed by imaging the insert surfaces at 10x magnification.

The Low-X inserts demonstrated significantly higher wear rates (mean 4.66 mg/million cycles) than the High-X inserts (mean 1.55 mg/million cycles, p < 0.001). Wear scars on the Low-X inserts were irregular and visibly deeper than those on the High-X inserts. The machining marks on the surface of the insert were also better preserved in the High-X insert wear scars. These results suggest that crosslinked PE can significantly reduce wear in TKR under physiologic conditions. This can result in reduced lysis and increased survivorship. Localized damage can cause catastrophic failure in polyethylene knee inserts. Therefore, further studies are necessary to evaluate wear under these conditions.


Orthopaedic Proceedings
Vol. 86-B, Issue SUPP_I | Pages 5 - 5
1 Jan 2004
Hermida J Patil S Chen P Colwell C D’Lima D
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This study measured polyethylene wear and correlated it with design features such as tibiofemoral conformity and contact areas.

Two femoral component designs were tested in a knee wear simulator. The femoral condyles of design A were flat-on-flat in the coronal plane, while those of design B were curved-on-curved. These femoral components were tested with two inserts. Insert PLI had a posterior lip, while insert C had a more curved sagital geometry, to improve stability in the anteroposterior direction. All components were tested for up to five million cycles in bovine serum lubricant. Triaxial forces were monitored to ensure that loading conditions were similar in all combinations tested. Gravimetric wear measurements were made at 500 000 cycle intervals. Contact stresses were measured using pressure sensitive film and dynamic finite element analysis.

Contact stresses were 22% higher for inserts tested with design A compared to design B. Sliding distance, sliding velocity, and patterns of crossing motion were found to be comparable between the two femoral designs. Inserts tested with design A wore significantly more (mean 10.9 mg/million cycles) than design B (mean 5.71 mg/million cycles, p < 0.001). No appreciable differences were found between wear rates of insert PLI and insert C.

Component design can have a significant impact on polyethylene wear rate. Careful control of kinematic and loading conditions allowed for comparison between specific design features. Increase in tibio-femoral contact area led to reduction of contact stresses, which was reflected in the reduced wear rate.