Are PEEK-on-Ceramic Bearings an Option for Total Disc Arthroplasty? An In Vitro Tribology Study.

BACKGROUND: Most contemporary total disc replacements (TDRs) use conventional orthopaedic bearing couples such as ultrahigh-molecular-weight polyethylene (polyethylene) and cobalt-chromium (CoCr). Cervical total disc replacements incorporating polyetheretherketone (PEEK) bearings (specifically PEEK-on-PEEK bearings) have been previously investigated, but little is known about PEEK-on-ceramic bearings for TDR. QUESTIONS/PURPOSES: (1) What is the tribologic behavior of a PEEK-on-ceramic bearing for cervical TDR under idealized, clean wear test conditions? (2) How does the PEEK-on-ceramic design perform under impingement conditions? (3) How is the PEEK-on-ceramic bearing affected by abrasive wear? (4) Is the particle morphology from PEEK-on-ceramic bearings for TDRs affected by adverse wear scenarios? METHODS: PEEK-on-ceramic cervical TDR bearings were subjected to a 10 million cycle ideal wear test based on ASTM F2423 and ISO 181912-1 using a six-station spine wear simulator (MTS, Eden Prairie, MN, USA) with 5 g/L bovine serum concentration at 23° ± 2° C (ambient temperature). Validated 1 million cycle impingement and 5 million cycle abrasive tests were conducted on the PEEK-on-ceramic bearings based, in part, on retrieval analysis of a comparable bearing design as well as finite element analyses. The ceramic-on-PEEK couple was characterized for damage modes, mass and volume loss, and penetration and the lubricant was subjected to particle analysis. The resulting mass wear rate, volumetric wear rate, based on material density, and particle analysis were compared with clinically available cervical disc bearing couples. RESULTS: The three modes of wear (idealized, impingement, and abrasive) resulted in mean mass wear rates of 0.9 ± 0.2 mg/MC, 1.9 ± 0.5 mg/MC, and 2.8 ± 0.6 mg/MC, respectively. The mass wear rates were converted to volumetric wear rates using density and found to be 0.7 ± 0.1 mm3/MC, 1.5 ± 0.4 mm3/MC, and 2.1 ± 0.5 mm3/MC, respectively. During each test, the PEEK endplates were the primary sources of wear and demonstrated an abrasive wear mechanism. Under idealized and impingement conditions, the ceramic core also demonstrated slight polishing of the articulating surface but the change in mass was unmeasurable. During abrasive testing, the titanium transfer on the core was shown to polish over 5 MC of testing. In all cases and consistent with previous studies of other PEEK bearing couples, the particle size was primarily < 2 µm and morphology was smooth and spheroidal. CONCLUSIONS: Overall, the idealized PEEK-on-ceramic wear rate (0.7 ± 0.1 mm3/MC) appears comparable to the published wear rates for other polymer-on-hard bearing couples (0.3-6.7 mm3/MC) and within the range of 0.2 to 1.9 mm3/MC reported for PEEK-on-PEEK cervical disc designs. The particles, based on size and morphology, also suggest the wear mechanism is comparable between the PEEK-on-ceramic couple and other polymer-on-ceramic orthopaedic couples. CLINICAL RELEVANCE: The PEEK-on-ceramic bearing considered in this study is a novel bearing couple for use in total disc arthroplasty devices and will require clinical evaluation to fully assess the bearing couple and total disc design. However, the wear rates under idealized and adverse conditions, and particle size and morphology, suggest that PEEK-on-ceramic bearings may be a reasonable alternative to polyethylene-on-CoCr and metal-on-metal bearings currently used in cervical TDRs.

Development and evaluation of a dual-output vocal cord vibration switch for persons with multiple disabilities.

PURPOSE: A novel dual-output vocal cord vibration switch is described and evaluated for two individuals with severe motor disabilities and complex communication needs. An evaluative case series was performed to compare the new device to its previously proposed single-output counterpart in terms of accuracy, speed and user fatigue. METHOD: In an ABAB design, participants were followed for up to 16 days as they performed several image matching tasks. Custom-written software recorded the average time taken to match each item, as well as the total number of selection errors. Participants reported their fatigue levels using a modified five-point Borg scale. RESULTS: For one participant, the dual-output version significantly decreased task completion time and selection error (p > 0.05, Kolmogorov-Smirnov test), thus improving her communication ability. For the second participant, no significant differences were observed between the single- and dual-output systems. Fatigue associated with illness and self-reported stress with school exams were identified as limiting factors. CONCLUSIONS: The dual-output vocal cord vibration switch provides a promising new alternative for individuals with severe and multiple disabilities who are able to hum or produce vocalizations. Both participants expressed desire to continue using the device for educational and entertainment purposes.

Predator detection in Lymnaea stagnalis.

Laboratory-reared Lymnaea are capable of detecting and responding to the scent of a crayfish predator. The present investigation is a first attempt to characterize multiple stress-related behavioural responses resulting from predator detection and to depict the neurophysiological correlates of one of these illustrated behaviours. Snails respond to crayfish effluent (CE) by increasing the following behaviours: aerial respiration, exploratory/searching phase and sensitivity to the shadow-elicited full-body withdrawal response. In contrast, when snails detect CE they decrease both their righting response time when dislodged from the substratum and their basal cutaneous oxygen consumption. Interestingly, basal heart rate does not change in response to CE exposure. Finally, we directly measured the activity of the neuron that initiates aerial respiratory behaviour, RPeD1, in semi-intact preparations. Naïve snails exposed to CE prior to recording demonstrated both a significantly reduced spontaneous firing rate and fewer bouts of bursting activity compared with non-exposed snails. These data show that laboratory-reared Lymnaea that have never experienced a natural predator are still capable of detecting and responding to the presence of a historically sympatric predator. These data open a new avenue of research, which may allow a direct investigation from the behavioural to the neuronal level as to how an ecologically relevant stressful stimulus alters behaviour.