BACKGROUND:Collared cementless hip stems have demonstrated a reduced incidence of periprosthetic femoral fractures compared to collarless counterparts. Many fractures occur during implantation, when collarless stems are seated to achieve press-fit, causing critical tensile strains in the femur. Collared stems can limit excessive seating and subsidence through calcar-collar contact. This study aimed to explain the clinically observed smaller fracture rates with collared stems by comparing strain distributions during implantation and loading between collared and collarless stems. It was hypothesized that collared stems distribute applied forces through both the collar and stem, increasing compressive axial and shear strains, allowing higher load tolerance. METHODS:Seven collared and seven collarless stems were implanted with constant velocity (0.1 mm/s) in porcine femurs until failure. Two human cadaveric femurs were tested as proof of concept. Shear, axial compressive and tangential tensile strains were compared alongside fracture patterns, subsidence and forces. FINDINGS:Collared stems in porcine femurs resisted approximately twice as much force until failure occurred (collared: 4187 N, collarless: 1980 N; p < 0.001), with similar tangential tensile strains (1 % to 1.4 % p = 0.805) and subsidence of 1.6 mm for collarless and 1.1 mm for collared stems at different failure forces (p = 0.288). Axial compressive strain was heavily increased by 1147 % with collared stems (collared: 1.2 %, collarless: 0.1 %; p = 0.026). Human femurs exhibited similar trends. INTERPRETATION:During loading, the collar prevents periprosthetic femoral fractures by increasing axial compressive strains instead of causing critical excessive tangential tensile strains (hoop strains) that can result in fractures.

BACKGROUND:Cortical contact of the stem is a key determinant for primary stability in cementless revision hip arthroplasty. This matched-pair cadaveric study evaluated whether implant designs that increase cortical contact improve primary fixation of cementless revision stems METHODS: Ten paired femora received either a tapered monoblock RECLAIM™ stem with advanced spline (RAS) geometry or an identically sized prototype solid stem. Axial seating and rotation were recorded using dynamic image correlation during implantation. Specimens were cyclically loaded up to 200 % body weight, after which torque-to-failure was measured FINDINGS: Both designs showed mean micromotion <50 μm during cyclic loading, consistently favourable for osseointegration. However, solid stems rotated more during implantation than RAS stems (3.6 ± 5.0° vs. 0.5 ± 0.6°, p = 0.088). The cortical contact area of the solid stems was 43.6 % larger than RAS stems (p < 0.001) without an impact on superior fixation: Axial subsidence and rotation during cyclic loading did not differ significantly between the designs but two solid-stem specimens fractured under high loading. Torque-to-failure of the RAS stems was 38.7 ± 7.5 nm, exceeding that of solid stems by 24.3 % (p = 0.032) INTERPRETATION: These findings suggest that maximising circumferential contact by implant design alone cannot compensate for irregularities of the femoral canal. Whereas solid stems wedge firmly only at the cost of positioning accuracy and increased fracture risk, the thin splines of the RAS design engage the cortex progressively, guide the implant to the planned depth, and augment torsional resistance without excessive press-fit. Tapered stems with cortex-indenting splines improve primary stability in cementless revision hip arthroplasty more effectively than increasing contact area alone.

BACKGROUND:In cementless total hip arthroplasty stems are inserted into the bone by mallet blows. Surgeons are not instructed to adjust the force of their blows to differences among patients especially with regard to weight. Whether this is linked to complications is yet unknown. This study investigated factors that could affect the mechanical behavior of the femur-tissue system. METHODS:Four cadavers were subject to two total hip arthroplasties by the same surgeon - one side via a lateral approach and the contralateral side via a direct anterior approach. A mass-spring-damper model was used to replicate the mechanical response of the femur-tissue system of the cadavers and make them comparable. FINDINGS:The mechanical response in terms of mass-spring-damper parameters differed between the approaches (lateral: 16.5 kg, 29.7 N/mm, 467.1 Ns/m; direct anterior: 11.5 kg, 41.7 N/mm, 553.0 Ns/m). INTERPRETATION:Common metal-on-metal mallet blows in surgery are very short and mostly excite high frequencies that are clearly above the natural frequency of the femur-tissue system. Those overcritical force impulses make the stem slide into the femur before the bone can even start moving. Hence, the individual mechanical behavior of the femur-tissue system can be disregarded provided that the force is applied with very short blows. This needs to be considered for any attempt to replace the mallet in the operation theater (e.g. automated surgical impaction tools) or to modify the mallet (e.g. alternative tip material). Furthermore, it may provide guidance on the fixation of femurs in in vitro testing to mimic surgical reality.

Background: Femoral stem impaction in total hip arthroplasty is commonly performed by mallet blows on a metal impactor attached to the stem. Factors including the surgeon, the impactor, and the patient can influence the impaction. A wide range of impactors, varying in design and thus in mass and stiffness, are available. However, little is known about their influence on the force transmission and, consequently, about the proportion of the mallet force that ultimately reaches the implant. This study aimed to investigate the force transmission through the impactor for different impactor designs, while investigating different patient-specific femur-tissue systems in situ and in silico. Methods: The mallet and impactor forces of 9 consecutive blows on seated femoral stems were measured for 2 different approaches on each of 4 cadavers. The mallet-implant force transmission was calculated using a phenomenological model for 2 different impactor designs. Results: The attenuated force in the impactor achieved approximately 65% to 75% of the corresponding mallet peak force, regardless of cadaver or surgical approach. Measuring the force distant from the tip resulted in an overestimation of the transferred forces. Depending on impactor design just 24% to 47% of the applied mallet peak force reached the implant itself. Conclusions: The force transmission for overcritical mallet blows can be regarded as independent from patient- and approach-specific boundary conditions and primarily dependent on the impactor design. Surgeons must be aware of this relationship and exercise caution when using other or novel instruments to prevent intraoperative complications.

[This corrects the article DOI: 10.1016/j.artd.2025.101622.].

Background:Primary stability is of great importance for the longevity of the implant in cementless revision total hip arthroplasty, since instability is a major cause of rerevision. The purpose of this study was to evaluate the effect of an additional set of less prominent, wider splines added to an established conical stem design with sharp splines on axial stability in a model with significant proximal bone defects. Methods:Twenty fresh-frozen human femurs were implanted with either the established or the additional spline design, dynamically loaded and tested in a load-to-failure configuration. Cortical contact in the femoral canal after implantation was evaluated by superimposing computed tomography scans and 3-dimensional laser scans. Stem subsidence and micromotion were evaluated to assess primary stability. Results:Stems remained stable during cyclic loading of up to 200% body weight, except in bones with cortical bone mineral density below 1000 mgHA/mL. A significant reduction of more than 85% in stem subsidence (P = .040), axial micromotion (P = .007), and rotational micromotion (P = .010) was achieved with the new spline design. Load-to-failure testing exceeded 400% body weight. Conclusions:The new spline design increased the cortical contact which resulted in increased axial primary stability in this in vitro experiment. Bone mineral density as a measure of bone quality proved to be a decisive factor for achieving immediate postoperative stability. Further variations of the established stem designs could further improve the longevity of artificial joint replacements.

[This corrects the article DOI: 10.1371/journal.pone.0303682.].

Aims:Sufficient primary implant stability with minimal bone damage is one of the challenges for uncemented implant fixation to prevent periprosthetic fractures and implant loosening. A pilot study on a non-viscoelastic material (polyurethane foam) showed a reduced impaction force when using vibratory implant insertion. This study assessed the effectiveness of vibratory implant insertion compared to an established implant insertion method in physiological viscoelastic bone from porcine hips. Methods:Acetabular components were impacted line-to-line and into 1 mm nominal undersized cavities in porcine acetabula (n = 24 in total, n = 6 acetabula per group of study) using vibration (60 Hz) and 1 Hz (established) impaction methods. The impaction force, remaining polar gap, and lever-out moment were measured and compared between the impaction methods and different press-fits. Results:The vibratory impaction method produced almost 40% lower impaction forces at both press-fit levels. However, complete seating at the nominal press-fit of 1 mm was not achieved, and primary stability was lower for the vibratory impaction for either press-fit. Conclusion:Bone fracture risk due to high impaction forces could be reduced by vibrational implant insertion at the cost of a reduction in primary stability. The outcome of the vibratory impaction method in porcine bone was similar to a previous study using polyurethane foams, suggesting that the viscoelasticity of bone may not play a crucial role during press-fit implant impaction.

Aims Periprosthetic fracture and implant loosening are two of the major reasons for revision surgery of cementless implants. Optimal implant fixation with minimal bone damage is challenging in this procedure. This pilot study investigates whether vibratory implant insertion is gentler compared to consecutive single blows for acetabular component implantation in a surrogate polyurethane (PU) model. Methods Acetabular components (cups) were implanted into 1 mm nominal under -sized cavities in PU foams (15 and 30 per cubic foot (PCF)) using a vibratory implant insertion device and an automated impaction device for single blows. The impaction force, remaining polar gap, and lever -out moment were measured and compared between the impaction methods. Results Impaction force was reduced by 89% and 53% for vibratory insertion in 15 and 30 PCF foams, respectively. Both methods positioned the component with polar gaps under 2 mm in 15 PCF foam. However, in 30 PCF foam, the vibratory insertion resulted in a clinically undesirable polar gap of over 2 mm. A higher lever -out moment was achieved with the consecutive single blow insertion by 42% in 15 PCF and 2.7 times higher in 30 PCF foam. Conclusion Vibratory implant insertion may lower periprosthetic fracture risk by reducing impaction forces, particularly in low -quality bone. Achieving implant seating using vibratory insertion requires adjustment of the nominal press -fit, especially in denser bone. Further preclinical testing on real bone tissue is necessary to assess whether its viscoelasticity in combination with an adjusted press -fit can compensate for the reduced primary stability after vibratory insertion observed in this study.

Increasing the stem size during surgery is associated with a higher incidence of intraoperative periprosthetic fractures in cementless total hip arthroplasty with fully coated tapered wedge stems, especially in femurs of Dorr type A. If in contrast a stem is implanted and sufficient primary stability is not achieved, such preventing successful osseointegration due to increased micromotions, it may also fail, especially if the stem is undersized. Stem loosening or periprosthetic fractures due to stem subsidence can be the consequence. The adaptation of an established stem design to femurs of Dorr type A by design modifications, which increase the stem width proximally combined with a smaller stem tip and an overall shorter stem, might reduce the risk of distal locking of a proximally inadequately fixed stem and provide increased stability. The aim of this study was to investigate whether such a modified stem design provides improved primary stability without increasing the periprosthetic fracture risk compared to the established stem design. The established (Corail, DePuy Synthes, Warsaw, IN, US) and modified stem designs (Emphasys, DePuy Synthes, Warsaw, IN, US) were implanted in cadaveric femur pairs (n = 6 pairs) using the respective instruments. Broaching and implantation forces were recorded and the contact areas between the prepared cavity and the stem determined. Implanted stems were subjected to two different cyclic loading conditions according to ISO 7206-4 using a material testing machine (1 Hz, 600 cycles @ 80 to 800 N, 600 cycles @ 80 to 1600 N). Translational and rotational relative motions between stem and femur were recorded using digital image correlation. Broaching and implantation forces for the modified stem were up to 40% higher (p = 0.024), achieving a 23% larger contact area between stem and bone (R2 = 0.694, p = 0.039) resulting in a four times lower subsidence during loading (p = 0.028). The slight design modifications showed the desired effect in this in-vitro study resulting in a higher primary stability suggesting a reduced risk of loosening. The higher forces required during the preparation of the cavity with the new broaches and during implantation of the stem could bare an increased risk for intraoperative periprosthetic fractures, which did not occur in this study.

The ideal stem size and stem position is important for the success of total hip arthroplasty, since it can affect early implant loosening and periprosthetic fractures (PPF). This study aimed to investigate how small deviations from the ideal stem size and position influences the PPF risk and primary stability. Six experienced surgeons performed preoperative templating based on which the benchmark size for each femur was determined. Consecutive implantations were performed in six cadaveric femur pairs-one side was implanted with an undersized stem followed by the benchmark size and the contralateral side with a benchmark size followed by an oversized stem (Corail, Depuy Synthes). Moreover, three different alignments (six varus, six neutral, six valgus-undersized) were compared using 18 femurs. Cortical strains during broaching and implantation were measured, and laser scans were used to determine final stem position. All specimens underwent dynamic loading. Primary stability was estimated from stem subsidence and pull-out forces. Templated stem size varied between surgeons (+/- 1 size; p = 0.005). Undersizing increased stem subsidence by 320% (p < 0.001). Oversized stems exhibited 52% higher pull-out forces (p = 0.001) and 240% higher cortical strains (p = 0.056). Cortex strains increased with varus alignment (R-2 = 0.356, p = 0.011) while primary stability decreased with valgus stem alignment (p = 0.043). Surgeons should be aware that small deviations from the ideal stem size and malalignments of the stem can significantly alter the mechanical situation and affect the success of their surgery.

The implantation of uncemented prostheses requires the application of sufficient forces to achieve a press-fit of the implant in the bone. Excessive forces have to be omitted to limit bone damage. Force measurements along the force transmission path between mallet and implant are frequently used to investigate this trade-off. Placing a load cell at a position of interest (PoI), which might be the implant bone interface or the head taper junction, is technically challenging or even impossible so that nearby positions are chosen. Thus, a certain inertia and stiffness remain between the PoI and the sensor, and consequently the measured dynamic forces differ from those at the PoI. This experimental and numerical study aimed to investigate the amount of force reduction along the transmission path while joining femoral heads to stem tapers. Forces were measured in vitro at the tip of the mallet, directly above the polymer tip of the impactor and below the stem taper. Springs and masses were used to represent the responding tissue of a patient. A semi-empirical numerical model of the force transmission path was developed and validated in order to simulate a larger range of responding tissue properties than experimentally possible and to investigate the influence of different surgical instruments. A distinct attenuation was observed since the peak forces at the impactor reached 35% of the applied peak forces and 21% at the stem taper, respectively. The force curves were replicated with a median root mean square error of 3.8% of the corresponding mallet blow for the impactor and 3.6% for the stem. The force measurement position and the used surgical instruments have a strong influence on the measured forces. Consequently, the exact measurement conditions with regard to sensor positioning and used surgical instruments have to be specified and hence only studies with similar setups should be compared to avoid misestimation of the forces at the PoI. The proposed dynamic numerical model is a useful tool to calculate the impact of the chosen or changed mechanical parameters prior to executing experiments and also to extrapolate the effect of changing the applied forces to the resulting forces at the PoI.

Introduction: In revision surgery, modular implant components allow the surgeon to tailor the characteristics of the implant to the bone situation. Relative motion can occur at the tapered modular connection, leading to fretting corrosion and subsequent biological reactions, particularly due to poor assembly and contamination of the tapered connection. The aim of this study was to demonstrate whether incomplete assembly and inadvertent contamination of the modular taper causes a change in junction strength. Material and methods: Modular taper junctions between the neck and the stem (n = 48) were divided into seven groups that differed with respect to contamination (native, contaminated, cleaned) and assembly conditions (secured, pre-tensioned and secured). Contamination was achieved by a combination of porcine bone particles and bovine blood. For each group, the number of rotations of the torque limiter while securing the conical connection was recorded. The implants were subjected to cyclic loading. DIC was used to determine neck rotation, micromotion and axial subsidence. Loosening torque of the locking screw and pull-off forces were measured as an equivalent of residual taper junction strength. Results: Contamination of the taper junction, especially in combination with improper assembly of the components, significantly increased the rotation (35.3 +/- 13.7 degrees vs. 2.4 +/- 4.4 degrees; p <0.001), micromotion (67.8 +/- 16.9 mu m vs. 5.1 +/- 12.1 mu m, p <0.001) and axial subsidence (-34.1 +/- 16.9 mu m vs. 4.3 +/- 10.9 mu m; p <0.001) of the neck relative to the stem. Conclusion: Intra-operatively, contamination of the taper surface can be identified by the need for multiple turns when tightening the locking screw. Correct cleaning with the new taper cleaning instrument and complete assembly with pre-tensioning may reduce the risk of early failure and fatigue fracture of the modular taper connection.

Aseptic loosening is one of the major reasons for re-revisions of cementless revision stems. Insufficient primary stability is associated with bone characteristics and the surgical process. This study aimed to investigate how femur morphology and preparation methods influence the primary stability of revision stems. The Femur morphology was described by the upper femoral curvature (UFC) and an individualized Dorr type classification based on the ratio between the canal-to-calcar ratio (CCR*) and the cortical index (CI*) introduced as the cortical-canal shape (CCS). Manual and powered reaming in combination with helical and straight reamers were used to prepare the bone cavity of 10 cadaveric human femur pairs. Forces during stem impaction were recorded (Reclaim, Depuy Synthes). Micromotion at the bone-implant interface during cyclic axial loading and torsional load to failure was determined. The CCS and impaction forces (R2 = 0.817, p < 0.001) or torsional strength (R2 = 0.577, p < 0.001) are inversely related. CCS did not correlate with micromotion during axial loading (R2 = 0.001, p > 0.999), but proximal femoral curvature did (R2 = 0.462, p = 0.015). Powered reaming and straight reamers led to an improved torsional strength (both: p = 0.043). The Individualized Dorr classification CCS and UFC allows a good estimation of the primary stability of revision stems. For severely curved Dorr type-C femurs, an alternative anchorage method should be considered clinically.

Background: Totally implantable central venous access port devices are crucial for intravenous application of chemotherapeutics and long-term therapy for chronic disease. Common complications include thrombosis and device fracture related to altered material properties through exposure in situ. This study exhibits whether uniaxial tensile properties (DIN 10555-3) of in vivo used catheters prove inferior to unused catheters.Material and methods: 5 unused, originally packed silicone catheters were cut into 6 segments of 50 mm: 3 segments each were cleaned via cleaning solution (n = 15) while 3 segments were left unattended (n = 15). Distal segments (50 mm) of long-term in vivo used silicone catheters were cleaned before testing (n = 33). Overall mechanical behavior was tested in a custom-made self-centering, torsion free carrier. Maximum force stress at failure, strain at failure and Young's modulus were determined and statistically analyzed.Findings: Unused catheters showed no significant difference in testing. in vivo used catheters exhibited 20% lower maximal force than unused catheters (p < 0.001), strain at break (p 0,001), and 7% higher elastic modulus (p = 0.004; power: 0.845). Due to a constant cross section area, stress at failure was proportional to maximum force (p < 0.001). Relation between determined parameters and dwell times was non-significant.Interpretation: In vivo long-term used silicone catheters showed significantly lower ultimate strength than unused ones. It is likely that in situ altering changes the mechanical properties of catheters and may potentially lead to failure.

Zusammenfassung Einführung In der Revisionschirurgie ermöglichen modulare Implantatkomponenten dem Chirurgen, die Eigenschaften des Implantats an die Knochensituation anzupassen. An der modularen Verbindung kann es zu Relativbewegungen kommen, die zu Abrieb und nachfolgenden biologischen Reaktionen führen, insbesondere aufgrund einer unvollständigen Montage und Kontamination der konischen Verbindung. Ziel dieser Studie war es, zu zeigen, ob eine unvollständige Montage und eine versehentliche Kontamination des modularen Konus eine Veränderung der Verbindungsstärke verursacht. Material und Methoden Modulare Konusverbindungen zwischen Hals und Schaft (n = 48) wurden in sieben Gruppen eingeteilt, die sich hinsichtlich des Verunreinigungsgrades (nativ, kontaminiert, gereinigt) und der Fügebedingungen (gesichert, vorgespannt und gesichert) unterschieden, und nach dem Fügen mit einer servohydraulischen Prüfmaschine zyklisch belastet wurden. Die Kontamination wurde durch eine Kombination aus Schweineknochenpartikeln und Rinderblut erreicht. Für jede Gruppe wurde zusätzlich die Anzahl der Umdrehungen des Drehmomentbegrenzers beim Sichern der Konusverbindung erfasst. Mittels digitaler Bildkorrelation wurden die Verdrehung des Halsteils, die Mikrobewegung und das axiale Setzen des Halsteils ermittelt. Anschließend wurden die Abzugskräfte als Maß für die verbleibende Verbindungsfestigkeit der Konusverbindung bestimmt. Ergebnisse Eine Verunreinigung der Konusverbindung, insbesondere in Kombination mit einer unsachgemäßen Montage der Komponenten, erhöhte signifikant die Rotation (35,3 ± 13,7° vs. 2,4 ± 4,4°; p <0,001), die Mikrobewegung (67,8 ± 16,9 μm vs. 5,1 ± 12,1 μm, p <0,001) und das axiale Setzen (‑34,1 ± 16,9 μm vs. 4,3 ± 10,9 μm; p <0,001) des Halses gegenüber dem Schaft. Schlussfolgerung Intraoperativ lässt sich eine Kontamination der Konusoberfläche daran erkennen, dass beim Festziehen der Sicherungsschraube mehrere Umdrehungen erforderlich sind. Eine korrekte Reinigung mit dem neuen Konusreinigungsinstrument und eine vollständige Montage mit Vorspannen der Komponenten können das Risiko eines frühzeitigen Versagens und eines Ermüdungsbruches der modularen Konusverbindung verringern.

Background: Patient-specific 3D-printed miniplates for free flap fixation in mandibular reconstruction were recently associated with enhanced osseous union. Higher mechanical strains resulting from these plates are discussed as reasons, but biomechanical studies are missing. This study aims to examine, whether patient-specific 3D-printed miniplates provide an increased interosteotomy movement (IOM) and lower stiffness compared with reconstruction plates. Methods: Polyurethane (PU) mandible and fibula models (Synbone AG, Malans, Schweiz) were used to simulate mandibular reconstruction with a one segment fibula flap equivalent. Osteosynthesis was performed using either four patient-specific 3D-printed miniplates (3D-Mini) or one patient-specific 3D-printed reconstruction plate (3D-Recon). Mastication was simulated using cyclic dynamic loading with increasing loads until material failure or a maximum load of 1000 N. Continuous IOM recording was carried out using a 3D optical tracking system (ARAMIS, Carl Zeiss GOM Metrology, Braunschweig, Germany).Findings: The averaged stiffness at a load of 100-300 N load did not differ between the groups (p = 0.296). There was a faster 1.0 mm vertical displacement in the 3D-Mini group (26 376 +/- 14 190 cycles versus 44 817 +/- 30 430 cycles, p = 0.018). The IOM were higher with miniplate fixation in the distal gap (p = 0.040). In the mesial gap, there was no significant difference between the groups (p = 0.160). Interpretation: Fixation with patient-specific 3D-printed miniplates results in higher mechanical strains. Lower rates of pseudarthrosis, as seen in clinical studies, might be caused by this phenomenon. Surgeons should evaluate the primary use of 3D-printed miniplates in mandibular reconstruction due to advantages of intraoral plate removal alongside safe osteosynthesis.

The aim of the study was to investigate whether the primary stability of press-fit acetabular components can be improved by altering the impaction procedure. Three impaction procedures were used to implant acetabular components into human cadaveric acetabula using a powered impaction device. An impaction frequency of 1 Hz until complete component seating served as reference. Overimpaction was simulated by adding ten strokes after complete component seating. High-frequency implantation was performed at 6 Hz. The lever-out moment of the acetabular components was used as measure for primary stability. Permanent bone deformation was assessed by comparison of double micro-CT (µCT) measurements before and after impaction. Acetabular component deformation and impaction forces were recorded, and the extent of bone-implant contact was determined from 3D laser scans. Overimpaction reduced primary acetabular component stability (p = 0.038) but did not significantly increase strain release after implantation (p = 0.117) or plastic deformations (p = 0.193). Higher press-fits were associated with larger polar gaps for the 1 Hz reference impaction (p = 0.002, R2 = 0.77), with a similar trend for overimpaction (p = 0.082, R2 = 0.31). High-frequency impaction did not significantly increase primary stability (p = 0.170) at lower impaction forces (p = 0.001); it was associated with smaller plastic deformations (p = 0.035, R2 = 0.34) and a trend for increased acetabular component relaxation between strokes (p = 0.112). Higher press-fit was not related to larger polar gaps for the 6 Hz impaction (p = 0.346). Overimpaction of press-fit acetabular components should be prevented since additional strokes can be associated with increased bone damage and reduced primary stability as shown in this study. High-frequency impaction at 6 Hz was shown to be beneficial compared with 1 Hz impaction. This benefit has to be confirmed in clinical studies.

INTRODUCTION:In revision surgery, modular implant components allow the surgeon to tailor the characteristics of the implant to the bone situation. Relative motion can occur at the tapered modular connection, leading to fretting corrosion and subsequent biological reactions, particularly due to poor assembly and contamination of the tapered connection. The aim of this study was to demonstrate whether incomplete assembly and inadvertent contamination of the modular taper causes a change in junction strength.MATERIAL AND METHODS:Modular taper junctions between the neck and the stem (n = 48) were divided into seven groups that differed with respect to contamination (native, contaminated, cleaned) and assembly conditions (secured, pre-tensioned and secured). Contamination was achieved by a combination of porcine bone particles and bovine blood. For each group, the number of rotations of the torque limiter while securing the conical connection was recorded. The implants were subjected to cyclic loading. DIC was used to determine neck rotation, micromotion and axial subsidence. Loosening torque of the locking screw and pull-off forces were measured as an equivalent of residual taper junction strength.RESULTS:Contamination of the taper junction, especially in combination with improper assembly of the components, significantly increased the rotation (35.3 ± 13.7° vs. 2.4 ± 4.4°; p <0.001), micromotion (67.8 ± 16.9 μm vs. 5.1 ± 12.1 μm, p <0.001) and axial subsidence (‑34.1 ± 16.9 μm vs. 4.3 ± 10.9 μm; p <0.001) of the neck relative to the stem.CONCLUSION:Intra-operatively, contamination of the taper surface can be identified by the need for multiple turns when tightening the locking screw. Correct cleaning with the new taper cleaning instrument and complete assembly with pre-tensioning may reduce the risk of early failure and fatigue fracture of the modular taper connection.

BACKGROUND:Despite the success of primary total hip arthroplasty, the number of revisions remains high. Infection, aseptic loosening, periprosthetic fractures and dislocations are the leading causes of hip revision. Current revision stem designs feature a tapered body with circumferential placed longitudinal thin metal splines that cut into the femoral cortex of the diaphysis to provide axial and rotational stability. Modifications to the spline design may help improve primary stability in various bone qualities. The purpose of this study was to evaluate whether the rotational stability of a revision hip stem can be improved by an additional set of less prominent, wider splines in addition to the existing set of splines. It is hypothesized that the additional splines will result in greater cortical contact, thereby improving torsional strength.METHODS AND FINDINGS:The ultimate torsional strength of an established modular revision stem (Reclaim®, DePuy Synthes) was compared to a Prototype stem design with two sets of splines, differing in prominence by 0.25 mm. Five pairs of fresh-frozen human femurs (n = 10) were harvested and an extended trochanteric osteotomy was performed to obtain common bone defects in revision. Stems were implanted using successive droptower impacts to omit variability caused by mallet blows. The applied energy was increased from 2 J in 1 J increments until the planned implantation depth was reached or seating was less than 0.5 mm at 5 J impact. The ultimate torsional strength of the bone-to-implant interface was determined immediately after implantation. Image superposition was used to analyze and quantify the contact situation between bone and implant within the femoral canal. Cortical contact was larger for the Prototype design with the additional set of splines compared to the Reclaim stem (p = 0.046), associated with a higher torsional stability (35.2 ± 6.0 Nm vs. 28.2 ± 3.5 Nm, p = 0.039).CONCLUSIONS:A second set of splines with reduced prominence could be shown to improve primary stability of a revision stem in the femoral diaphysis in the presence of significant proximal bone loss. The beneficial effect of varying spline size and number has the potential to further improve the longevity of revision hip stems.