Custom-built constrained uniaxial and rotating hinge total knee replacement in cats: Clinical application, design principles, surgical technique, and clinical outcome.

OBJECTIVE: To describe the design principles and evolution, surgical technique, and outcome for custom constrained (uniaxial and rotating hinge) total knee replacement (TKR) in cats. STUDY DESIGN: Retrospective case series. ANIMALS: Nine cats with traumatic stifle luxation (n = 8) or severe distal femoral deformity (n = 1) were considered suitable candidates. METHODS: Cats that met eligibility criteria and received a custom TKR between 2009 and 2018 by a single surgeon were included in this case series. Three generations of implant were used. Implant positioning was assessed by postoperative orthogonal radiography. Functional outcome was determined by clinical assessment, owner interview, and a feline musculoskeletal pain index questionnaire. RESULTS: Median clinical follow-up time was 12 months (range, 4-41); follow-up time was increased to 29 months (range, 22-47) when results of functional questionnaires with owner were included. Median radiographic follow-up was 12 months (range, 4-25). One cat had a catastrophic outcome. Three cats had good outcomes, and five cats had excellent outcomes. CONCLUSION: Most cats treated with custom-built TKR achieved good to excellent outcomes. CLINICAL SIGNIFICANCE: Custom TKR is a viable option for the treatment of severe pathologies of the feline stifle. Additional research is required to fully evaluate implant suitability.

Treatment of a large osseous defect in a feline tarsus using a stem cell-seeded custom implant.

The aim of this study is to describe the treatment of an infected segmental bone defect in a cat using a novel, custom-designed titanium implant seeded with adipose-derived stem cells (AdMSCs) to facilitate osseous ingrowth and preserve limb function. Large bone defects occur secondary to trauma, infection, or neoplasia and often result in amputation. We established a novel autologous AdMSC-impregnated trabecular metal spacer made using 3D printing, to bridge the distal tibia and metatarsal bones in the left pelvic limb of a cat that had previously undergone right pelvic limb amputation. Six months postoperatively, there was radiographic evidence of bone growth and implant integration. A titanium spacer seeded with AdMSCs successfully encouraged bone ingrowth in a large defect site and successfully preserved limb function. However, further studies are needed to justify the use of differentiated stem cell impregnated mesh as a framework to bridge large bone defects.

Effect of impact assembly on the interface deformation and fretting corrosion of modular hip tapers: An in vitro study.

Wear and corrosion at the modular head-neck junction has been recognised to be a potential clinical concern, with multiple reports on adverse local tissue reactions and subsequent early failure of metal-on-metal hip replacements. Furthermore, reports on head-neck taper corrosion are also being described with conventional metal-on-polyethylene bearings. Manufacturing tolerances, surgical technique, non-axial alignment, material combination, high frictional torque and high bending moment have all been implicated in the failure process. There is limited guidance on the force of impaction with which surgeons should assemble modular hip prostheses. This study aims to investigate the effect of impaction force on the deformation and corrosion of modular tapers. Short neck tapers with high surface roughness (average Rz = 16.58 µm, Ra = 4.14µm) and long neck tapers with low surface roughness (average Rz = 3.82 µm, Ra = 0.81µm), were assembled with CoCrMo alloy heads (smooth finish) under controlled conditions with 2, 4 or 8 kN of impaction force. Material combinations tested included CoCrMo-head/CoCrMo-neck and CoCrMo-head/Ti-6Al-4V-neck. Assessment of surface deformation before and after impaction was made using surface profilometry. Measurement of fretting current during sinusoidal cyclic loading evaluated mechanically assisted corrosion for each assembly load during short-term cyclic loading (1000-cycles) and long-term cyclic loading (5 million-cycles). Deformation on head and neck tapers increased with assembly load. Fretting currents during short term simulation testing showed significantly lower currents (p < 0.05), in 8 kN assemblies when compared to 2 and 4 kN, especially for the short-rough tapers. Long-term simulator testing demonstrated a progressive reduction in fretting corrosion for samples impacted with 4 and 8 kN; however, this reduction was greater for samples impacted at 8 kN even at the start of testing. Based on our results, surgeons could minimise mechanically assisted crevice corrosion by using higher impact loads when assembling the head to the stem in total hip arthroplasty. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:405-416, 2018.

Clinical relevance of corrosion patterns attributed to inflammatory cell-induced corrosion: A retrieval study.

In vitro studies have shown that human osteoclasts can corrode stainless steel and titanium leading to the production of metal ions responsible for inflammatory reactions. Moreover, traces of cellular activities on metal orthopaedic explants have recently been reported as inflammatory cell-induced (ICI) corrosion being the result of the cells sealing on the metal surfaces and releasing reactive oxygen species (ROS) through Fenton-like reactions. The extent and clinical relevance of this phenomenon has yet to be understood. We analysed a cohort of 100 CoCr alloy hips collected at our retrieval centre; we performed macroscopic and microscopic screening and used statistical analysis to correlate our findings with implant and clinical variables. We found that 59% of our implants had evidence of surface damage consistent with what has previously been described as cell-induced corrosion. There was a significant association between the patterns and aseptic loosening for the ASR modular (r = -0.488, p = 0.016) and the Durom modular (r = 0.454, p = 0.026). This is the largest implant retrieval study to examine the phenomena of so-called ICI corrosion and is the first to investigate its clinical relevance. We recommend further work to determine the role of cells in the damage patterns observed. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 155-164, 2017.

Fretting corrosion behavior of nitinol spinal rods in conjunction with titanium pedicle screws.

Untypical corrosion damage including erosions combined with the build-up of titanium oxide as a corrosion product on the surface of explanted Nitinol spinal rods in the areas where it was in contact with titanium pedicle screw head is reported. It was suggested that Nitinol rods might have inferior fretting corrosion resistance compared with that made of titanium or CoCr. Fretting corrosion of Nitinol spinal rods with titanium (Ti6Al4V) pedicle screws were tested in-vitro by conducting a series of potentiostatic measurements of the peak-to-peak values of fretting corrosion current under bending in a 10% solution of calf serum in PBS. The test included Nitinol rods locked in titanium pedicle screws of different designs. Performance of commercially available titanium (Ti6Al4V) and CoCr spinal rods was also investigated for a comparison. Corrosion damage observed after the in-vitro tests was studied using SEM and EDAX analysis and was compared with patterns on Nitinol rods retrieved 12months after initial surgery. Metal ions level was measured in the test media after in-vitro experiments and in the blood and tissues of the patients who had the rods explanted. The results of this study revealed that Nitinol spinal rods locked in Ti pedicle screws are susceptible to fretting corrosion demonstrating higher fretting corrosion current compared with commercially used Ti6Al4V and CoCr rods. On the surface of Nitinol rods after in-vitro tests and on those retrieved from the patients similar corrosion patterns were observed. Improved resistance to fretting corrosion was observed with Nitinol rods in the in-vitro tests where pedicle screws were used with a stiffer locking mechanism. Since the development of the localized corrosion damage might increase the risk of premature fatigue failure of the rods and result in leaching of Ni ions, it is concluded that Nitinol rods should not be used in conjunction with Ti pedicle screws without special protection especially where the design provides a high degree of mobility to the rods.

Stability at the half pin-frame interface on external fixation constructs.

A mechanical study investigating the use of two different methods (grub and bolt screws) to secure external fixation half pins to circular frames. A four part experiment: (1) Grub and bolt screws were used to secure half pins in Taylor Spatial frames. Loosening torques were measured using a calibrated torque wrench. (2) Using universal testing machine (UTM), axial loading was applied to establish thresholds for loosening in grub and bolt screw constructs. (3) We established the application torque to produce failure at the head-driver interface using these two methods. (4) Grub and bolt screw constructs were created controlling torque. Using UTM, axial loading was applied to establish thresholds for loosening. Statistical analysis was conducted using SPSS v20.0.0. (1) Higher torque is employed when bolt rather than grub screws is used to secure half pins on Rancho cubes (p < 0.05). (2) Loading threshold for loosening is higher in bolt screw constructs when the torque applied to secure the constructs is not controlled (p < 0.05). (3) Torque required for failure at the head-driver interface was 5.3 Nm for grub screws and 9.9 Nm for bolts. (4) Loading threshold for loosening is higher in grub screw constructs when the same torque was applied to secure them (p < 0.05). Bolt screws can be employed to secure the half pin-frame interface. They offer good stability and reduce failure at the head-driver interface. Further research is needed to determine the mechanical properties of such constructs in vivo.

An In Vitro Comparison of the Primary Stability of 2 Tapered Fluted Femoral Stem Designs.

BACKGROUND: Proximal bony deficiencies present a biomechanical challenge to achieving primary stability in revision hip arthroplasty. Long tapered fluted stems have been engineered to span these defects but concerns of early subsidence are well documented. This work aimed primarily to investigate the issue of subsidence with this design using a cadaveric model. A secondary aim was to compare the stability of 2 versions of this design. METHODS: Seven pairs of cadaveric femora were obtained, dual emission x-ray absorpitometry scanned, with calibration radiographs taken for digital templating. Each bone was potted according to the ISO standard for fatigue testing and a Paprosky type 3 defect was simulated. The established cone-conical Restoration Modular (Stryker) system and a novel design with a chamfered tip and flute configuration (Redapt, Smith & Nephew) were examined. Movement at the stem-bone interface was measured using radiostereometric analysis and micromotion transducers. RESULTS: All restoration stems and 85% of the Redapt stems achieved stability by recognized criteria, micromotion < 150 µm and migration less than 2 mm. A Fisher exact test comparing the proportion of stems which were stable or unstable was not significant, P = .055. Mean axial subsidence (SD) was 0.17 mm (0.32) and 0.1 mm (0.131) for the Restoration and Redapt stems respectively. CONCLUSION: This study has demonstrated minimal subsidence in the immediate post-operative period using tapered fluted stems. Both designs achieved excellent stability despite simulation of Paprosky type 3 bony defects in the cadaveric model. This geometry appears satisfactory for use in revision surgery in the presence of significant proximal bony deficiencies.

Influence of stem type on material loss at the metal-on-metal pinnacle taper junction.

The clinical importance of material loss at the head-stem junction is unknown. Comparison of retrievals with different stem types can provide the opportunity to understand the importance of the taper junction. This was a case-control study involving 20 retrieved 36 mm metal-on-metal Pinnacle (DePuy) hips that were paired with either a Corail (n = 10) or S-ROM (n = 10) stem. The median head taper material loss rate for the Corail group was 0.238 (0.0002-2.178) mm(3)/year and was significantly greater than the S-ROM group (p = 0.042), which had a median material loss rate of 0.132 (0.015-0.518) mm(3)/year. The only significant difference between the groups was the stem taper roughness and length: this was rougher and shorter for the Corails. Long and smooth stem taper designs are preferred when used in conjunction with metal heads.

Analysis of retrieved growth guidance sliding LSZ-4D devices for early onset scoliosis and investigation of the use of nitinol rods for this system.

STUDY DESIGN: Analysis of volumetric wear loss of retrieved growth guidance sliding devices LSZ-4D for treatment of early onset scoliosis and laboratory in vitro wear test for comparison of wear resistance of alloys Nitinol, Ti, and cobalt chromium (CoCr). OBJECTIVE: To evaluate quantitatively the amount of wear debris from the sliding LSZ-4D device and to investigate the potential of using Nitinol for replacing Ti alloys in spinal instrumentation. To do that, wear resistance of Nitinol, Ti, and CoCr was compared. SUMMARY OF BACKGROUND DATA: There are little data regarding the amount of wear debris associated with growth guidance sliding devices for patients with early onset scoliosis and the wear resistance of superelastic Nitinol compared with Ti and CoCr. METHODS: Volumetric wear loss was measured on LSZ-4D devices made from titanium alloy Ti6Al4V and each consisted of 2 rectangular section (6 × 4 mm) rods and 40 ± 8 fixture elements (20 ± 4 hooks and 20 ± 4 clips) retrieved from 3 patients (implantation period, 3.5-5.8 yr). Images of wear scars were taken on Bruker interferometer microscope and incorporated into MATLAB software. Wear resistance of Nitinol, Ti, and CoCr was studied using reciprocation pin-on-disk wear test in bovine serum at 37°C ± 1°C. RESULTS: The volume wear rate of LSZ-4D device was found to be 12.5 mm per year from which 5 mm³ per year is the wear debris of the rod and 7.5 mm per year is the contribution of fixtures. Wear resistance of Nitinol is 100 times higher than that of Ti and comparable with that of CoCr. CONCLUSION: Application of wear-resistant coatings on Ti components in growth guidance sliding devices for the treatment of early onset scoliosis will be useful. High wear resistance of Nitinol combined with its superelastic and shape memory properties could make application of Nitinol rods for spinal instrumentation beneficial.

Enhanced wear and corrosion in modular tapers in total hip replacement is associated with the contact area and surface topography.

Widespread concern exists about adverse tissue reactions after metal-on-metal (MoM) total hip replacement (THR). Concerns have also been expressed with wear and corrosion of taper junctions in THR. We report the effect of surface finish and contact area associated with a single combination of materials of modular tapers. In an in vitro test, we investigated the head/neck (CoCrMo/Ti) interface of modular THRs using commercially available heads. Wear and corrosion of taper surfaces was compared following a 10 million loading cycle. Surface parameters and profiles were measured before and after testing. Electrochemical static and dynamic corrosion tests were performed under loaded and non-loaded conditions. After the load test, the surface roughness parameters on the head taper were significantly increased where the head/neck contact area was reduced. Similarly, the surface roughness parameters on the head taper were significantly increased where rough neck tapers were used. Corrosion testing showed breaching of the passive film on the rough but not the smooth neck tapers. Thus, surface area and surface finish are important factors in wear and corrosion at modular interfaces.

Mechanical stability of a locked step-plate versus single compression screw fixation for medial displacement calcaneal osteotomy.

BACKGROUND: Reconstruction of a flatfoot commonly involves a calcaneal Medial Displacement Osteotomy (MDO) to correct hindfoot valgus in combination with soft tissue procedures. We compared fixation of an MDO using either a single, large cannulated screw versus a locking step-plate in load to failure in a cadaveric model. METHODS: Eight matched pairs of cadaveric limbs were loaded using a mechanical testing rig. Two pairs served as non-operated controls. The remaining paired limbs underwent a 10-mm MDO stabilized either with a single 7-mm screw or a step-plate with four locking screws. One pair was used as a pilot study and the remaining five pairs were loaded up to 4500 N to failure. RESULTS: In the five pairs loaded to failure, the median (with 95% CI) maximum force were 1779 N (1099-2312) and 826 N (288-1607) for the plate and screw, respectively (p = 0.043). With single screw fixation, the tuberosity fragment consistently failed by rotation and angulation into varus. With plate fixation, failure occurred as the screws cut through the internal surfaces of the tuberosity and body with no failure at the screw-plate interface. CONCLUSION: In this cadaveric model, a locked step-plate supported a significantly higher maximum force than a single large cannulated screw. CLINICAL RELEVANCE: The magnitude of the load supported by the locking step-plate suggests that allowing early weightbearing post-operation may be safe in clinical practice before union of the osteotomy.

Measurement of forces generated during distraction of growing-rods in early onset scoliosis.

AIM: To measure the forces applied during distraction of growing-rods in early onset scoliosis (EOS), aimed at developing a motorized elongation device. METHODS: A consecutive series of measurements were carried out to analyze the forces applied by the surgeon during distraction of single growing-rods in 10 patients affected by EOS (mean age 8.3 years; range 6 to 10 years) undergoing the first distraction 6 months following implantation of the rods. For each measurement, output from the transducer of a dedicated pair of distraction calipers was recorded at zero load status and at every 1 mm of distraction, up to a maximum of 12 mm for each of the two connected rods. RESULTS: Twenty measurements were obtained showing a linear increase of the load with increasing distraction, with a mean peak force of 485 N at 12 mm distraction and a single reading over 500 N. We did not observe bone fractures or ligament disruptions during or after rod elongations. There was one case of superficial wound infection in the cohort. CONCLUSION: The safe peak force carrying capacity of a motorized device for distraction of growing-rods is 500N.

An experimental glenoid rim strain analysis for an improved reverse anatomy shoulder implant fixation.

Loosening of glenoid components in TSA is a main cause of failure. In reverse anatomy TSA designs used for unstable joints, fixation is particularly demanding. Strains developed around the glenoid rim of biomechanical sawbone scapulae implanted with (a) the original fixed-fulcrum Bayley-Walker glenoid prosthesis in current clinical use, and (b) a revised version with conical cross-section, were compared. The conical shape of the revised design was hypothesized to produce greater strains in the glenoid rim than the original tapered screw design. The 2D strain field at three accessible locations around the rim of each scapula was measured with three-element rosette strain gauges for two types of simulated cancellous bone fill under applied physiologically relevant loads. The average strain energy densities around the rim for the conical design were greater than for the original design by a factor of 1.55-2.25 for all loading conditions. Results indicate that a significantly greater proportion of load was directed toward cortical bone in the conical design, thus promoting cortical bone loading.

Strain response of an instrumented intramedullary nail to three-point bending.

OBJECTIVES: An experimental biomechanical evaluation of an instrumented intramedullary nail (TriGen® META Nail, Smith&Nephew®) was undertaken. The objectives were two-fold. The first was to identify the most sensitive strain gauge positions and orientations on the nail, and the second was to demonstrate that the nail was capable of detecting changes in stiffness of the nail-bone composite. The function of the instrumented nail is to quantify fracture healing objectively and directly, and so to predict delayed repair or non-union 2 months before current methods. METHODS: Eight flat pockets were machined onto the surface of the nail and three strain gauges attached in each pocket. The instrumented nail was inserted into fourth generation biomechanical grade Sawbones® tibiae with three different fracture configurations as well as into a non-fractured bone. The nail-bone composite was loaded in three-point bending at five positions to determine the strain changes in each of the eight strain gauge pockets located along the length of the nail. To simulate callus in the simplest way and to increase the stiffness of the nail-bone composite, loops of duct tape in multiples of four were applied over the fracture locus. A three-point loading jig was used to obtain the change in strain with increasing stiffness. Relative displacement of the bone ends was quantified using radiostereometric analysis. RESULTS: There was no single position of greatest strain sensitivity for all fracture types. The greatest change in strain occurred when the strain gauge pocket and fracture line were closest. Applying the loading moment directly over the strain gauge pocket also maximised its sensitivity. The duct tape callus simulation showed that the instrumented nail was able to detect a change in stiffness of less than 4.1 Nm/°. CONCLUSIONS: It has been shown that the instrumented nail can detect physiologically relevant changes in stiffness, and so to provide a useful function as an objective monitor of fracture repair.