Mechanically assisted crevice corrosion (MACC) at the modular femoral head taper junction in total hip arthroplasty (THA) is a significant concern, influenced by a combination of design factors such as taper length, diameter, surface finish, and neck shaft angle. While each factor individually affects corrosion resistance, their interactions play a critical role in determining overall implant performance. This study aimed to systematically evaluate the interaction of key design factors on fretting and corrosion performance in modular femoral head tapers. A full-factorial Design of Experiments (DOE) approach was utilized, testing 16 unique taper configurations under cyclic loading conditions. Fretting currents were measured at various intervals, and post-test analysis included Scanning Electron Microscopy (SEM) and Energy-Dispersive Spectroscopy (EDS) to qualitatively assess surface damage and corrosion byproducts. Surface finish emerged as the most influential factor, with rough finishes significantly reducing fretting currents compared to smooth finishes. The combination of larger taper diameters and longer lengths, which enhance structural rigidity, also contributed to lower fretting currents. SEM and EDS analyses corroborated these findings, showing less deformation and corrosion product accumulation in configurations with rough surface finishes and enhanced rigidity. This study highlights the importance of accounting for interactions among design factors when evaluating modular taper performance. Increased structural rigidity and roughened surface finishes were associated with reduced fretting currents and corrosion under the conditions tested (Ti-CoCr pairing), suggesting that these design characteristics may help mitigate mechanically assisted crevice corrosion and extend implant longevity in total hip arthroplasty.

Diffuse idiopathic skeletal hyperostosis (DISH) is characterized by “flowing” ossifications that form along the anterolateral aspect of contiguous vertebrae. While prior literature has identified that DISH patients are susceptible to spinal injury, the DISH-specific differences in mechanical response of human spines to applied bending have not been quantified. Therefore, the purpose of this work was to assess the effect of DISH on range of motion, stiffness, and fracture tolerance under flexion/extension loading. Seven human cadaveric spines were dissected into specimens composed of three functional spinal units (3FSUs), spanning levels T1–T4, T5–T8, and T9–T12. For each 3FSU specimen, the presence of bridging DISH ossification was determined. Ten DISH-affected and eleven normal specimens were evaluated under flexion/extension range of motion followed by extension to failure/system limits. DISH-affected specimens had significantly less range of motion compared to normal specimens (1.8 deg vs. 3.2 deg, p = 0.0125) and were significantly stiffer (5.5 N·m/deg vs. 2.9 N·m/deg, p = 0.0167). All DISH-affected specimens fractured during extension to failure, whereas five normal specimens did not fracture under the test loading. Hazard ratio analyses indicated that DISH-affected specimens are four times more likely to fracture compared to normal specimens under a given torque and sixteen times more likely for a given extension angle. The normal specimen median energy at fracture was significantly greater than the DISH-affected group (301 Nm·deg vs. 81 Nm·deg, p = 0.006). This study demonstrates a significant increase in the injury potential of DISH spines during bending loading and provides the first quantitative support for clinical observations of compromised fracture tolerance within the DISH population. Future work classifying the severity of DISH in the context of mechanical response may assist in radiological identification of otherwise asymptomatic patients with increased potential for injury.

Total ankle replacement (TAR) has become an effective treatment for end‐stage ankle osteoarthritis. Multiple factors, including patient characteristics, surgical technique, alignment, and bearing surfaces, influence TAR survivorship. Polyethylene (PE) fatigue is a key consideration in improving outcomes. This study establishes a novel, clinically relevant testing protocol incorporating varus‐valgus rotation to simulate polyethylene fatigue failures observed in mobile‐bearing total ankle replacements. Using this robust methodology, we evaluated the impact of oxidation and antioxidant stabilization on ultrahigh‐molecular‐weight polyethylene (UHMWPE) performance in a mobile bearing implant application. A six‐degree‐of‐freedom simulator was used to iteratively adjust loading parameters (1500–3000 N, −4° to +8° flexion‐extension, ±5° axial rotation, and ±3° or ±8° varus‐valgus rotation at 37 ± 3°C in 20 g/L bovine serum) until clinically observed midline fractures were replicated. Oxidation levels were measured by Fourier‐transform infrared spectroscopy per ASTM F2102. This validated loading protocol was then applied to conventional (25 kGy GUR 1020) and vitamin E‐stabilized (75 kGy GUR 1020‐E) UHMWPE inserts and tested to visible fracture or a 3‐million‐cycle runout. Post‐test fractographic analysis identified crack initiation sites. Conventional aged UHMWPE demonstrated fatigue failure under varus‐valgus rotation (OI = 2.59 ± 1.11) but no failure without rotation. Vitamin E‐stabilized UHMWPE showed no fatigue failure after 3 million cycles, even under varus‐valgus rotation (OI = 0.23 ± 0.02). Fractography revealed fractures originating at the trough and propagating with cyclic loading. Oxidation significantly reduces polyethylene fatigue life, and varus‐valgus rotation exacerbates this effect in mobile bearing TAR implants. Antioxidant‐stabilized UHMWPE showed promising resistance to fatigue and oxidation. These findings support the role of antioxidant stabilization in improving TAR performance, and the protocols developed here provide a framework for assessing the safety of alternative materials.

Wear particles invariably form due to contact and friction between articulating surfaces in orthopedic prosthetic joint replacements. Polycarbonate urethane (PCU) has shown low wear rates and invoked minimal local biological response to wear debris in various orthopedic applications. However, controlled preclinical studies have not yet studied the biological response to PCU particles in synovial joints. This study aims to evaluate the biological response to mostly submicron-sized PCU wear particles in synovial joints in a rabbit model representing a worst-case scenario. PCU and ultra-high-molecular-weight-polyethene (UHMWPE) particles were generated in vitro, and particle characterization was performed using scanning electron microscopy (SEM) images. Fifteen New Zealand white rabbits, divided into three groups, received bilateral injections in the knee joint with 10 mg/mL PCU, UHMWPE particles, or saline (all 0.2 mL). After 3 months, the biological response in the joint was evaluated by histopathological reactivity scoring. The generated PCU and UHMWPE wear particles were mainly in the biologically active size range with an average equivalent circle diameter (ECD) of 0.31 μm (±0.48) and 6.99 μm (±16.32), respectively. There was a minimal to non-existing biological response (score ≤ 0.5) to PCU (0.5 ± 1.0), UHMWPE particles (0.6 ± 1.3) and saline (0.0 ± 0.0). Also, the wear particles did not disperse from the injection site. The results of this study support the use of PCU as a bearing surface in orthopedic prosthetic joint replacements by indicating that even in the likelihood that wear particles are generated, they are not likely to trigger a strong inflammatory response.

Background: A novel, lumbar total joint replacement (TJR) design has been developed to treat degeneration across all three columns of the lumbar spine (anterior, middle, and posterior columns). Thus far, there has been no in vitro studies that establish the preclinical safety profile of the vitamin E-stabilized highly crosslinked polyethylene (VE-HXLPE) lumbar TJR relative to historical lumbar anterior disc replacement for the known risks of wear and impingement faced by all motion preserving designs for the lumbar spine. Questions/Purpose: In this study we asked, (1) what is the wear performance of the VE-HXLPE lumbar TJR under ideal, clean conditions? (2) Is the wear performance of VE-HXLPE in lumbar TJR sensitive to more aggressive, abrasive conditions? (3) How does the VE-HXLPE lumbar TJR perform under impingement conditions? Method: A lumbar TJR with bilateral VE-HXLPE superior bearings and CoCr inferior bearings was evaluated under clean, impingement, and abrasive conditions. Clean and abrasive testing were guided by ISO 18192-1 and impingement was assessed as per ASTM F3295. For abrasive testing, CoCr components were scratched to simulate in vivo abrasion. The devices were tested for 10 million cycles (MC) under clean conditions, 5 MC under abrasion, and 1 MC under impingement. Result: Wear rates under clean and abrasive conditions were 1.2 ± 0.5 and 1.1 ± 0.6 mg/MC, respectively. The VE-HXLPE components demonstrated evidence of burnishing and multidirectional microscratching consistent with microabrasive conditions with the cobalt chromium spherical counterfaces. Under impingement, the wear rates ranged between 1.7 ± 1.1 (smallest size) and 3.9 ± 1.1 mg/MC (largest size). No functional or mechanical failure was observed across any of the wear modes. Conclusions: Overall, we found that that a VE-HXLPE-on-CoCr lumbar total joint replacement design met or exceeded the benchmarks established by traditional anterior disc replacements, with wear rates previously reported in the literature ranging between 1 and 15 mg/MC. Clinical Relevance: The potential clinical benefits of this novel TJR design, which avoids long-term facet complications through facet removal with a posterior approach, were found to be balanced by the in vitro tribological performance of the VE-HXLPE bearings. Our encouraging in vitro findings have supported initiating an FDA-regulated clinical trial for the design which is currently under way.

Background: Little is known about retrieved zirconia platelet toughened alumina (ZPTA) wear particles from ceramic-on-ceramic (COC) total hip arthroplasty. Our objectives were to evaluate clinically retrieved wear particles from explanted periprosthetic hip tissues and to analyze the characteristics of in vitrogenerated ZPTA wear particles. Methods: Periprosthetic tissue and explants were received for 3 patients who underwent a total hip replacement of ZPTA COC head and liner. Wear particles were isolated and characterized via scanning electron microscopy and energy dispersive spectroscopy. The ZPTA and control (highly cross-linked polyethylene and cobalt chromium alloy) were then generated in vitro using a hip simulator and pinon-disc testing, respectively. Particles were assessed in accordance with American Society for Testing and Materials F1877. Results: Minimal ceramic particles were identified in the retrieved tissue, consistent with the retrieved components demonstrating minimal abrasive wear with material transfer. Average particle diameter from in vitro studies was 292 nm for ZPTA, 190 nm for highly cross-linked polyethylene, and 201 nm for cobalt chromium alloy. Conclusion: The minimal number of in vivo ZPTA wear particles observed is consistent with the successful tribological history of COC total hip arthroplasties. Due to the relatively few ceramic particles located in the retrieved tissue, in part due to implantation times of 3 to 6 years, a statistical comparison was unable to be made between the in vivo particles and the in vitro-generated ZPTA particles. However, the study provided further insight into the size and morphological characteristics of ZPTA particles generated from clinically relevant in vitro test setups.

Background: Larger head-to-neck ratio of dual mobility (DM) hip arthroplasties provide greater range of motion/less risk of dislocation, but raise concerns for high wear and friction. We measured in vitro, the wear rates of contemporary DM hips with highly cross-linked ultra high molecular weight polyethylene (UHWMPE), where it came from, and their frictional torques. Methods: Hip simulators were used to compare the wear of DM to fixed-bearing (FB) designs of 2 different implants. Each of 8 different configurations underwent millions of simulated walking cycle tests, some as full DM, some as FB controls, some DM with the outer-articulation deliberately immobilized, and some the inner. Wear and 3-dimensional-frictional torques were measured and friction independent of size was deduced. Results: The DM hips produced lower wear and friction-torque than the FB hips. The DM wear during walking gait comes mostly from the smaller inner articular surface. If the outer surface was immobilized, the wear and torque of the inner alone would be small, but the full DM (inner and outer free-to-move) wear and torque were smallest of all. Friction measurements expectedly showed larger hips having higher frictional torques, but the DM showed the lowest, again because its motion was mostly the smaller inner articulation; smaller than even a modern fixed-bearing hip. Conclusion: The DM hips appear to combine the benefits of greater range of motion and less impingement of larger hips, with the lower wear and friction of smaller FB hips, with some benefits compromised if the outer or inner articulations are immobilized. (c) 2023 Elsevier Inc. All rights reserved.

Fused filament fabrication (FFF, 3D printing) of polyetheretherketone (PEEK) has been gaining attention for its potential use in creating orthopaedic implants at or near the point-of-care. Understanding the mechanical properties and degree of anisotropy imparted on PEEK by the FFF process is vital for designing and predicting the behavior of structures created using this technique. In this study, the mechanical behavior of FFF PEEK is described. Solid PEEK prisms were additively manufactured with the same printing parameters used for creating porous PEEK. Cyclic compression testing and testing until failure were then performed, with loading oriented either parallel or perpendicular to the material layer direction. Mechanical properties including Young’s modulus, yield strength, and Poisson’s ratio were then calculated for each orientation. We found that loading perpendicular to the layer direction resulted in an elastic modulus of 2.34 ± 0.22 GPa and yield strength of 58.0 ± 1.2 MPa, while parallel loading resulted in a modulus of 2.20 ± 0.46 GPa and yield strength of 68.0 ± 8.1 MPa. The degree of elastic modulus anisotropy was relatively low, with a difference of 6.2% between the values in each orientation. The difference in yield strength between orientations was 14.8%, with the xy-direction having significantly greater yield strength. Using scanning electron microscopy, we observed that the failed specimens had layer splitting occurring cleanly along the interface between layers. The results of this study provide fundamental information on the behavior of 3D printed PEEK which may serve as groundwork for future studies regarding the material.

Additive manufacturing (AM) is a potential application for polyetheretherketone (PEEK) spinal interbody fusion cages, which were introduced as an alternative to titanium cages because of their biocompatibility, radiolucency and strength. However, AM of PEEK is challenging due to its high melting temperature and thermal gradients. Although fused filament fabrication (FFF) techniques have been shown to 3D print PEEK, layer delamination was identified in first generation FFF PEEK cages [1]. A standard cage design [2] was 3D printed with a second generation FFF PEEK printer. The effect of changing layer cooling time on the FFF cages' mechanical strength was investigated by varying nozzle sizes (0.2 mm and 0.4 mm), print speeds (1500 and 2500 mm/min), and the number of cages printed in a single build (1, 4 and 8). To calculate the porosity percentage, FFF cages were micro-CT scanned prior to destructive testing. Mechanical tests were then conducted on the FFF cages according to ASTM F2077 [2]. Although altering the cooling time of a layer was not able to change the failure mechanism of FFF cages, it was able to improve cages' mechanical strength. Printing a single cage per build was associated with a higher ultimate load than printing multiple cages per build. Regardless of the cage number printed per build, cages printed with bigger nozzle diameter achieved higher ultimate load compared to cages printed with smaller nozzle diameter. Printing with a bigger nozzle diameter resulted in less porosity, which might have an additional benefit on the interlayer delamination failure mechanism.

Due to its excellent thermomechanical properties, polyaryletheretherketone (PEEK) and its composites have been subjected to intensive scrutiny as bearing materials since the 1980s for industrial and aerospace applications. Biotribology refers to the study of wear under biologically relevant conditions. The simplest and more widely accepted model for basic biotribological simulation of candidate biomaterials is the multidirectional pin-on-disk test. This chapter reviews the published biotribology literature for PEEK biomaterials, including PEEK composites, from a basic science and engineering perspective, gleaned from pin-on-disk testing.

To improve the longevity of total hip replacements (THR), it is necessary to prevent wear of the ultra-high molecular weight polyethylene (UHMWPE) bearing, as wear debris can cause osteolysis and a...

Retrieving and analyzing medical devices from revision or removal surgeries are critical activities for understanding device failures. However, the role of retrieval analysis should not be limited to the investigation of individual failures and subsequent root cause analysis. Implementation of retrieval programs that operate throughout the commercial life of devices can be a means by which manufacturers inform product development, augment regulatory submissions, evaluate and mitigate risks, implement continuous improvement, and develop new designs or product enhancements. Understanding how retrieval analysis can contribute to the Total Product Life Cycle of a device will enhance a manufacturer's ability to address regulatory requirements and U.S. Food and Drug Administration inquiries, identify and respond to unanticipated failure modes, evaluate successful in vivo performance of its devices, and to improve the quality, safety, effectiveness, and performance of new devices. We will review some of the basic scientific and regulatory requirements that apply to devices in the context of retrievals. We will also examine the role of retrieval analysis during device development within the design control framework and will provide guidelines for implementing a formal program that will allow manufacturers to realize the benefits of retrieval analysis throughout the Total Product Life Cycle of a medical device.

We report on an innovative surface grafting to highly crosslinked (HXLPE) bearing for THA using a biocompatible-phospholipid-polymer poly (2-methacryloyloxyethyl phosphorylcholine) (PMPC). Such hyd...

Taper corrosion has been suggested as a possible contributor to in vivo disassociation of modular connections in total hip arthroplasty (THA) systems, but this relationship has not been explored experimentally. This study assessed whether in vivo taper corrosion decreases the strength of the head-stem connection, and compared these taper characteristics between clinically revised devices and cadaver retrievals. One hundred nine (109) femoral stems retrieved with an attached cobalt-chrome (CoCr) head were identified in a collection of THA retrievals: 93 from revision surgery and 16 from cadaver donors. After the explants were cleaned, the force used to disassemble each head-stem pair was recorded using a mechanical test frame with custom fixtures in accordance with ISO 7206-10. Taper corrosion was assessed using a four-point semi-quantitative method. Femoral disassembly force was positively associated with stem taper damage (rho = 0.26, p = 0.007) but not significantly related to head taper damage (rho = 0.14, p = 0.153). There was no difference in femoral disassembly force between revision and cadaver retrievals. Revision retrievals exhibited greater damage than cadaver retrievals at both the head (odds ratio [OR] = 0.23, p = 0.002) and stem (OR = 0.06, p = 0.001) tapers. The results of the present study do not support the hypothesis that corrosion weakens the taper junction between the head and stem of modular femoral components. The findings from the taper damage assessment of cadaver controls may suggest a greater prevalence of corrosion in components requiring revision surgery.

To improve the longevity of total hip replacements (THR), it is necessary to prevent wear of the ultra-high molecular weight polyethylene (UHMWPE) bearing, as wear debris can cause osteolysis and aseptic loosening. Highly cross-linked UHMWPE reduces wear, sometimes stabilized with vitamin E to preserve its mechanical properties and prevent oxidative degeneration. An extra novel solution has been grafting the surface of UHMWPE with poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC). This treatment uses a hydrophilic (wettable) phospholipid polymer to improve lubrication and reduce friction and wear of the bearing material. We set out to test the wear and friction of ceramic-on-polyethylene (COP) THRs that had the PMPC surface treatment, or left untreated for control. Four groups of UHMWPE bearings were tested against identical 40mm ceramic heads (zirconia-toughened alumina). The UHMWPE bearings were highly cross-linked with/without vitamin E (HXL Vit. E: 125 kGy radiation dose / HXL: 75 kGy). In each group, half underwent the PMPC treatment (n = 3 for all four groups). Testing was conducted on an AMTI hip simulator for 10 million walking cycles of ISO-14242-1, at 1 Hz, with diluted bovine serum (30 g/L protein concentration) as lubricant, at 37oC, and with fluid absorption errors corrected with active soak controls. Using a previously published method, frictional torques and a frictional factor around three orthogonal axes about the femoral head were measured/computed, by data processing of the measurements of a 6-DOF load cell on each station of the hip simulator. Such friction measurements and stops for specimen weighing were carried out at regular intervals throughout the wear test. The HXL liners without and with the PMPC treatment wore at 5.86±0.402 mg/Mc and 1.70±1.36 mg/Mc, respectively (p=0.013) (Fig. 1). The HXL Vit. E liners without and with the PMPC treatment wore at 2.14±0.269 mg/Mc and 0.736±0.750 mg/Mc, respectively (p=0.035). The wear rates of the untreated HXL and HXL Vit. E liners were significantly different (p=0.0002) but no difference in wear rate was found between the two PMPC treated groups (p=0.179), although, as mentioned above, the PMPC treatment very significantly reduced wear in each case. The ceramic femoral heads showed little wear (weight loss) themselves. In general, the THRs showed decreasing friction over the 10 Mc, with the PMPC types showing a slight increase in friction towards the end of the test (Fig. 2). PMPC HXL liners showed the lowest friction factor (0.022±0.001) which was significantly lower (p PMPC successfully reduced both the friction and the wear in these COP THRs during this extended 10 Mc test. This likely would translate to improved implant longevity in patients. For any figures or tables, please contact authors directly (see Info u0026 Metrics tab above).

Background Context Total disc arthroplasty is an alternative therapy to spinal fusion for the treatment of neck or low back pain and is hypothesized to reduce the risk of disease progression to the adjacent spinal levels. Radiographic and retrieval analyses of various total disc replacements (TDRs) have shown evidence of impingement damage. Impingement of TDRs can occur when the device reaches the limits of its functional range of motion, causing contact between peripheral regions of the device. Purpose Impingement can be associated with increased wear and mechanical damage; however, impingement conditions are not simulated in current standardized mechanical bench test methods. This study explored the test conditions necessary to apply clinically relevant impingement loading to a lumbar TDR in vitro. Study Design An experimental protocol was developed and evaluated using in vivo retrievals for qualitative and quantitative validation. Methods Retrieval analysis was conducted on a set of 11 size 3 retrieved Charité devices using American Society for Testing and Materials F561 as a guide. The impingement range of motion was determined using a combination of modeling and experiments, and was used as an input in vitro testing. A 1-million cycle in vitro test was then conducted, and the in vitro samples were characterized using methods similar to the retreived devices. Results All in vitro tested samples exhibited impingement regions and damage patterns consistent with retrieved devices. Consistent with the retrievals, the impingement damage on the rim was a combination of abrasive wear and plastic deformation. Micro computed tomography (microCT) was used to quantitatively assess rim damage due to impingement. Rim penetration was statistically lower in the retrievals when compared with both in vitro groups. Rim elongation was comparable among all groups. The simulated-facet group had statistically greater angular rim deformations than the retrieval group and the no-facet group. Conclusions Results demonstrate that clinically relevant impingement seen on mobile bearings of lumbar TDRs can be replicated on the bench.

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.

BACKGROUND:Creating appropriately-sized, lethal isotherms during cryoablation of renal tumors is critical in order to achieve sufficiently-sized zones of cell death. To ensure adequate cell death in target treatment locations, surgeons must carefully select the type, size, location, and number of probes to be used, as well as various probe operating parameters.OBJECTIVE:The current study investigates the effects of probe type, operating pressure, and clinical method on the resulting sizes of isotherms in an in vitro gelatin model.METHOD:Using a total of four cryoprobes from two manufacturers, freeze procedures were conducted in gelatin in order to compare resulting sizes of constant temperature zones (isotherms). The effects of certain procedural parameters which are clinically adjustable were studied.RESULTS:Test results show that the sizes of 0 °C,-20 °C and -40 °C isotherms created by similarly-sized probes from two different manufacturers were significantly different for nearly all comparisons made, and that size differences resulting from changing the operating pressure were not as prevalent. Furthermore, isotherm sizes created using a multiple freeze procedure (a ten minute freeze, followed by a five minute passive thaw, followed by another ten minute freeze) did not result in statistically-significant differences when compared to those created using a single freeze procedure in all cases.CONCLUSION:These results indicate that selection of the probe manufacturer and probe size may be more important for dictating the size of kill zones during cryoablation than procedural adjustments to operating pressures or freeze times.

At a nanometer length scale, UHMWPE is a semicrystalline polymer, consisting of an amorphous matrix reinforced by stiffer, crystalline lamellae. UHMWPE composites can be engineered at a micro- or nanometer length scale by blending polymer powder resin with micro- or nanoparticles and fibers before consolidation. UHMWPE fibers provide another route to fabricating composites, as they can be woven, knitted, or nonwoven into sheets or plies forming two- and three-dimensional structures with direction-dependent properties. In addition, porous UHMWPE can be created as either an intermediate or final step in the production of open-cell, closed-cell, and reinforced composites. Thus, UHMWPE can play the role of matrix and fiber in a broad range of composite materials, depending upon the application. UHMWPE fibers are widely used in ballistic armor, and industrial and consumer applications. Examples are ropes and lines in sailing and offshore, nets in commercial fishing and cargo and knitted textiles for cut resistant gloves (Figure 22.1). The strong, flexible, and durable properties of the UHMWPE fibers are also attractive for medical applications, and UHMWPE fiber has become the gold standard for high-strength orthopedic sutures in the meantime. Other potential medical applications are being explored. This chapter presents the latest advancements in UHMWPE fiber technology and its use in medical applications.

All-polymer bearings involving polyetheretherketone (PEEK) have been proposed for orthopaedic applications because they may reduce stress shielding, reduce weight of the implants, reduce wear and risk of osteolysis, and prevent release of metal ions by replacing the metal articulating components. Little is known about the biotribology of all-polymer PEEK bearings, including the effects of cross-shear, which are relevant for implant longevity, especially in the hip, and increased temperature that may affect lubricant proteins and, hence, lubrication in the joint.