Introduction

Uncemented press-fit cups provide bone fixation in primary total hip replacement (THR). However, sometimes screws are needed to achieve primary stability of the socket. We analyzed biomechanical factors related to press-fit in seven cup designs and assessed whether screw use provides similar loosening rates to those of the press-fit technique.

INTRODUCTION

It is generally accepted that strong hammering is necessary for the press fit fixation of a joint prosthesis. In this regard, large stress must remain within bone tissues for a long period. This residual stress is, however, some different from the feasible mechanical stimuli for bone tissues because that is stationary, continuous and directed from within outward unlike physiological conditions. The response on this residual stress, which may induce the disorder of the fixation of implant, has not been discussed, yet.

In the present study, we designed an experimental method to exert a stationary load from inside of a femur of a rat by inserting a loop spring made from a super elastic wire of titanium alloy. Response of the femur was assessed by bone morphology mainly about the migration of the wire into the bone twelve weeks after the implantation.

THA in patients with acetabular bone defects is associated with a high risk of dislocation. Dual mobility (DM) cups are known to prevent and treat chronic instability. The aim of this study was to evaluate the dislocation rate and survival of jumbo DM cups. This was a retrospective, continuous, multicenter study of all the cases of jumbo DM cup implantation between 2010 and 2017 in patients with acetabular bone loss (Paprosky 2A: 46%, 2B: 32%, 2C: 15% and 3A: 6%). The indications for implantation were revisions for aseptic loosening of the cup (n=45), aseptic loosening of the femoral stem (n=3), bipolar loosening (n=11), septic loosening (n=10), periprosthetic fracture (n=5), chronic dislocation (n=4), intraprosthetic dislocation (n=2), cup impingement (n=1), primary posttraumatic arthroplasty (n=8), and acetabular dysplasia (n=4). The jumbo cups used were COPTOS TH (SERF), which combines press-fit fixation with supplemental fixation (acetabular hook, two superior flanges with one to four screws, two acetabular pegs). A bone graft was added in 74 cases (80%). The clinical assessment consisted of the Harris hip score. The primary endpoint was surgical revision for aseptic acetabular loosening or the occurrence of a dislocation episode. In all, 93 patients were reviewed at a mean follow-up of 5.3 ± 2.3 years [0, 10]. As of the last follow-up, the acetabular cup had been changed in five cases: three due to aseptic loosening (3.2%) and two due to infection (2.1%). The survivorship free of aseptic loosening was 96.8%. Three patients (3%) suffered a dislocation. At the last follow-up visit, the mean HSS scores were 72.15, (p < 0.05). Use of a jumbo DM cup in cases of acetabular bone defects leads to satisfactory medium-term results with low dislocation and loosening rates

During revision total knee arthroplasty (rTKA), proximal tibial bone loss is frequently encountered and can result in a less-stable bone-implant fixation. A 3D printed titanium alloy (Ti6Al4V) revision augment that conforms to the irregular shape of the proximal tibia was recently developed. The purpose of this study was to evaluate the fixation stability of rTKA with this augment in comparison to conventional cemented rTKA. Eleven pairs of thawed fresh-frozen cadaveric tibias (22 tibias) were potted in custom fixtures. Primary total knee arthroplasty (pTKA) surgery was performed on all tibias. Fixation stability testing was conducted using a three-stage eccentric loading protocol. Static eccentric (70% medial/ 30% lateral) loading of 2100 N was applied to the implants before and after subjecting them to 5×103 loading cycles of 700 N at 2 Hz using a joint motion simulator. Bone-implant micromotion was measured using a high-resolution optical system. The pTKA were removed. The proximal tibial bone defect was measured. One tibia from each pair was randomly allocated to the experimental group, and rTKA was performed with a titanium augment printed using selective laser melting. The contralateral side was assigned to the control group (revision with fully cemented stems). The three-stage eccentric loading protocol was used to test the revision TKAs. Independent t-tests were used to compare the micromotion between the two groups. After revision TKA, the mean micromotion was 23.1μm ± 26.2μm in the control group and 12.9μm ± 22.2μm in the experimental group. There was significantly less micromotion in the experimental group (p= 0.04). Prior to revision surgery, the control and experimental group had no significant difference in primary TKA micromotion (p= 0.19) and tibial bone loss (p= 0.37). This study suggests that early fixation stability of revision TKA with the novel 3D printed titanium augment is significantly better then the conventional fully cemented rTKA. The early press-fit fixation of the augment is likely sufficient for promoting bony ingrowth of the augment in vivo. Further studies are needed to investigate the long-term in-vivo fixation of the novel 3D printed augment

Background. Cementless acetabular cups rely on press-fit fixation for initial stability; an essential pre-requisite to implant longevity. Impaction is used to seat an oversized implant in a pre-prepared bone cavity, generating bone strain, and ‘grip’ on the implant. In certain cases (such as during revision) initial fixation is more difficult to obtain due to poorer bone quality. This increases the chance of loosening and instability. No current study evaluates how a surgeon's impaction technique (mallet mass, mallet velocity and number of strikes) may be used to maximise cup fixation and seating. Questions/purposes. (1) How does impaction technique affect a) bone strain & fixation and b) seating in different density bones? (2) Can an impaction technique be recommended to minimize risk of implant loosening while ensuring seating of the acetabular cup?. Methods. A custom drop tower was used to simulate surgical strikes, seating acetabular cups into a synthetic bone model (Fig. 1). Strike velocity (representing surgeon strike level) and drop mass (representing mallet mass) were varied through representative low, medium and high levels. Polar gap between the implant and bone was measured using optical tracking markers. Strain gauges were used to measure acetabular rim strain. Following seating, cup pushout force was measured in a materials testing machine. Both measurements were used to quantify the level of fixation of the implant for two conditions: For the first, the cup was optimally seated (moving no more than 0.1mm on the previous strike, representing ideal conditions); For the second the cup was impacted 10 times (excessively impacted). Repeats (N = 5) were conducted in low and high density bone; a total of 180 tests. Results. For ideally impacted cups, increasing mallet mass and velocity improved fixation and reduced polar gap. However a phenomenon of bone strain deterioration was identified if an excessive number of strikes were used to seat a cup, resulting in loss of implant fixation. This effect was most severe in low density bone (Fig. 2). For high strike velocity and mallet mass, each excessive strike halved the measured bone strain (78 ± 7 με/strike). This reduced fixation strength from 630 ± 65 N (optimally seated) to just 49 ± 6 N at 10 strikes (Fig. 3). Discussion. These results identify a possible mechanism of loss of implant stability with excessive acetabular impaction. A high mallet mass with low strike velocity resulted in satisfactory fixation (442 ± 38 N) and polar gap (1 ± 0.1 mm) whilst minimizing the fixation deterioration due to excessive mallet strikes. Extreme caution must be exercised to avoid excessive impaction high velocity strikes in low density bone for any mallet mass. Conclusion & Clinical relevance. As it may be difficult for a surgeon to accurately infer when an implant is optimally seated, this study informs surgeons of the effects of different impaction techniques, particularly in lower density bones. For any figures or tables, please contact the authors directly

Durable humeral component fixation in shoulder arthroplasty is necessary to prevent painful aseptic loosening and resultant humeral bone loss. Causes of humeral component loosening include stem design and material, stem length and geometry, ingrowth vs. ongrowth surfaces, quality of bone available for fixation, glenoid polyethylene debris osteolysis, exclusion of articular particulate debris, joint stability, rotator cuff function, and patient activity levels. Fixation of the humeral component may be achieved by cement fixation either partial or complete and press-fit fixation. During the past two decades, uncemented humeral fixation has become more popular, especially with short stems and stemless press fit designs. Cemented humeral component fixation risks difficult and complicated revision surgery, stress shielding of the tuberosities and humeral shaft periprosthetic fractures at the junction of the stiff cemented stem and the remaining humeral shaft. Press fit fixation may minimise these cemented risks but has potential for stem loosening. A randomised clinical trial of 161 patients with cemented vs. press fit anatomic total shoulder replacements found that cemented fixation of the humeral component provided better quality of life, strength, and range of motion than uncemented fixation but longer operative times. Another study found increased humeral osteolysis (43%) associated with glenoid component loosening and polyethylene wear, while stress shielding was seen with well-fixed press fit humeral components. During reverse replacement the biomechanical forces are different on the humeral stem. Stem loosening during reverse replacement may have different factors than anatomic replacement. A systemic review of 41 reverse arthroplasty clinical studies compared the functional outcomes and complications of cemented and uncemented stems in approximately 1800 patients. There was no difference in the risk of stem loosening or revision between cemented and uncemented stems. Uncemented stems have at least equivalent clinical and radiographic outcomes compared with cemented stems during reverse shoulder arthroplasty. Durable humeral component fixation in shoulder arthroplasty is associated with fully cemented stems or well ingrown components that exclude potential synovial debris that may cause osteolysis

Advances in military surgery have led to significant numbers of soldiers surviving with bilateral above knee amputations. Despite advances in prosthetic design and high quality rehabilitation not all amputees succesfully ambulate. Five patients (10 stumps) with persisting socket fit issues were selected for osseointegration (OI) using a transcutaneous prosthesis with press-fit fixation in the residual femur. Prior to surgery all five were primarily/exclusively wheelchair users. Follow up was from 7 to 25 months (mean 12.2). There were no deaths, episodes of sepsis or osteomyelitis. There was one proximal femoral fracture secondary to a fall. One stump required soft tissue refashioning. Cellulitis needing oral antibiotics occurred in four cases. Functional improvement occurred in all cases with all currently primarily prosthetic users, the majority all day users. Three patients are still completing rehabilitation. Six minute walk tests (SMWT) improved by a mean of 20%. Three are now graded mobility SIGAM F (normal gait) and two SIGAM D-b (limited terrain; with one stick). This cohort suggests that OI may have a role in the treatment of military blast amputees. A larger scale clinical evaluation is planned in the UK blast related amputee population to further establish the benefits and risks of this technique

INTRODUCTION. The cup component of modern resurfacing systems are often coated creating a cementless press-fit fixation in the acetabulum based on surgical under-reaming, also enabling osseoconduction/integration. Due to the higher density of cortical bone along the antero-superior and postero-inferior regions of the acetabulum, the greatest forces occur between the anterior and posterior columns of the pelvis. This produces pinching of the implant that can result in deformation of the cup. Metal shell/modularpress-fit acetabular cups are susceptible to substantial deformation immediately after implantation. This deformation may affect the lubrication, producing point loading and high friction torques between the head and the cup that increase wear and may lead to head clamping and subsequent cup loosening. We sought to test a novel ceramic on ceramic (CoC) hip resurfacing system that should allay any concerns with the Adverse Reaction to Metal Debris associated with metal on metal (MoM) resurfacing devices. AIM. We sought to quantify the deformation of a novel CoC hip-resurfacing cup after implantation, using a standard surgical technique in a cadaveric model, and compare to the MoM standard. We also assessed if the design clearances proposed for this CoC hip resurfacing implant are compatible with the measured deformations, allowing for an adequate motion of the joint. METHODS. The pelvis from four fresh frozen cadavers were placed into the lateral position. One surgeon with extensive experience in hip resurfacing surgery (JH) prepared all the pelvises for implantation using a posterior approach to the joint and sequential reaming of the acetabulum to 1mm below the implant outer diameter. The acetabulum components were then impacted into the prepared pelvis. We used four ceramic and four metal implants of equal and varying size. (2 × (40/46mm, 44/50mm, 50/56mm, 52/58mm)). The acetabulum cup bearing surface diameter and deformation was measured using a GOM-ATOS optical high precision 3D scanner. 3-Dimensional measurements were taken pre-implantation, immediately after and at 30 minutes following implantation. Two techniques were used to analyse the 3D images: by maximum inscribed diameter and by radial segments. These were compared to the known articulating surface clearance values. RESULTS. The diameter of the cups in both metal and ceramic systems was reduced after implantation when analysing by maximum inscribed diameter and by radial segments. This deformation was maintained at 30 minutes. We can infer there is no significant bone stress relaxation effect following implantation. On ceramic cups, the deformation was larger in larger sizes. However, the 44/50 (the second smallest cup) deformed the least. Despite this, the difference in deformation between these two sizes is minimal. The deformation of sizes 50/56 and 52/58 was equivalent. For the metal cups, there was not a clear correlation between the cup size and the deformation. The largest cup size had the same deformation as the smallest size. CONCLUSIONS. The deformation following implantation of the cup component in a ceramic acetabulum resurfacing behave similarly to a metal implant. Cup deformation measured after implantation is minimal when compared to the minimum design clearance in both systems

Periprosthetic fractures present several unique challenges including gaining fixation around implants, poor bone quality and deciding on an appropriate treatment strategy. Early. With the popularity of cementless stems in primary total hip arthroplasty (THA) we have seen a concomitant rise in the prevalence of intra-operative and early post-operative fractures of the femur. While initial press-fit fixation is a requirement for osseointegration to occur, there is a fine balance between optimising initial stability and overloading the strength of the proximal femur. Hence, the risk of intra-operative fractures is intimately related to the design of the femoral component utilized (metaphyseal engaging, wedge shaped designs having the highest risk) and the strength of the bone that it is inserted into (elderly females being at highest risk). These fractures typically are associated with a loose femoral component and require revision to a stem that gains primary fixation distally. We have found a high risk of complications and problems when treating these fractures in the early post-operative period with a high risk of infection, heterotopic ossification and the requirement for subsequent surgery. Late. The Vancouver Classification is based on the location of the fracture, the fixation of the implant and the quality of the surrounding host bone. The most common pitfall in treatment is mistaking a B2 fracture (stem loose) for a B1 (stem stable); treatment of a loose implant with ORIF alone will necessarily fail

Introduction. Good outcomes in reverse shoulder arthroplasty (RSA) rely in part on stability of the humeral component. Traditionally humeral components have been cemented, however there has been recent interest in press-fit fixation of humeral components in RSA. Lateralization of the head center in RSA can impart larger moments on the humeral component than for anatomic reconstructions, increasing the importance of distal humeral canal preparation for implant stability. To date, the primary stability of any type of press-fit humeral prosthesis has been largely unexplored. The goal of this study is to evaluate the effect of over-reaming the distal humeral canal in a press-fit humeral component in RSA. Methods. Computed tomography (CT) data of the shoulder were obtained from 55 shoulders. Images were segmented to produce digital models of the humerus. Humeral components for RSA (2mm diameter size increments) were sized and placed per the surgical technique, including preparation of the humerus with the appropriate reamers (1mm increments). Finite element models for each specimen were created with heterogeneous bone properties derived from the CT scan. Pressfit between the bone and stem was resolved to quantify the initial contact pressure on the stem; each stem was then loaded at 566N oriented 20° lateral and 45° anterior. Overall motion of the stem was measured, as well as interfacial micromotion in the porous coating region (Fig. 1). The effect of line-to-line (L2L) reaming and over-reaming by 1 mm was evaluated using an unpaired Student's t-test, with significance defined at p<0.05. Results. Across all specimens, stem sizes 8 (n=3), 10 (n=25), 12 (n=20), 14 (n=2), and 16 (n=1) were used. Stem motion ranged from approximately 250–750μm; micromotion remained under 300μm (Fig. 2). Stem motion was significantly less for L2L reaming as compared to over-reaming for both size 10 (p=.008) and size 12 (p=.002) stems; micromotion was significantly less for size 12 (p=.002) stems. L2L reaming to a larger diameter stem resulted in significantly reduced stem motion (average 390μm versus 530μm, p<.001) and micromotion (average 53μm versus 135μm, p=.001) than over-reaming and using a smaller diameter stem. Stem rotation following L2L reaming was generally below 0.5°, and exceeded 0.75° when over-reaming. Discussion and Conclusion. Reaming of the humeral canal directly impacts the stability of humeral stems in RSA. Even with satisfactory proximal press-fit, over-reaming enables increased rotation of the stem under functional loading prior to cortical engagement, and results in increased micromotion. In cases in which the reamer and stem offerings result in over-reaming, L2L reaming to the next larger stem significantly reduces stem motion and micromotion. However, reaming up also removes distal cortical bone, and thus the strength of the prepared humerus must be considered. In conclusion, line-to-line reaming significantly reduces the micromotion of humeral stems as compared to over-reaming

Femoral knee implants have promising outcomes, although some high-flex designs have shown rather high loosening rates (Han et al., 2007). In uncemented implants, it is vital to limit micromotions at the implant-bone interface, to facilitate secondary fixation through bone ingrowth (kienapfel et al., 1999). Hence, it is essential to investigate how micromotions of different uncemented implants are affected by various loading conditions when a range of bone qualities as a patient-related factor is applied. Using finite element (FE) analysis, we simulated implant-bone interface micromotions during four consecutive cycles of normal gait and squat movements. An FE model of a distal femur was generated based on calibrated CT-scans, after which Sigma® and LCS® Cruciate-Retaining Porocoat® components (DePuy Synthes, Leeds, UK) were implanted. Using a frictional contact algorithm (µ=0.95), an initial press-fit fixation was simulated, which was previously validated against experimental data. The micromotions were calculated by tracking the projection of implant nodes on the bone surface excluding overhang area. The applied loading patterns were based on discretized simulations, providing incremental loads for each activity based on implant-specific kinematics, which was derived from Orthoload database using inverse dynamics (Fitzpatrick et al., 2012). This provided the opportunity to calculate incremental micromotions, but also the resulting micromotions for each single cycle, for both activities. In addition, the percentage of implant surface area with resulting micromotions less than a defined threshold was calculated. Regardless of the type of loading, in all simulations, the predicted micromotions were highest in the first cycle, suggesting settling of the implant during initial cycle. The Sigma®implant displayed a 30% larger area with micromotions below the threshold of 5 microns, for both loading conditions (Fig. 1A). The highest micromotions occurred at the anterior flange, regardless of type of activity or design. Squatting had a more detrimental effect on the primary stability, with smaller areas of low micromotions as compared to the gait load (Fig. 1B). Bone stiffness had a minor effect, which was more apparent for squatting (Fig. 1B). We found acceptable low ranges of micromotions in both implant designs, although demanding activities such as squatting generated higher motions. In addition, LCS® experienced higher micromotions, probably caused by the smaller contact area at bone-implant interface compared with Sigma®. Nevertheless, the predicted micromotions were all below the clinically relevant threshold for bone ingrowth (<40 microns) (kienapfel et al., 1999). Furthermore, our simulated settling behavior stresses the necessity for simulating multiple loading cycles, rather than just a single cycle. The effect of bone stiffness was evident, but only to a limited extent. The main current limitation of our study is the utilization of an elastic material model for the bone which is probably the reason to predict a low range of micromotions. We are planning to make the material model more realistic, by including plasticity and viscoelastic bone behavior

Protrusio acetabuli (arthrokatadysis or Otto pelvis) is a relatively rare condition associated with secondary osteoarthritis of the hip. Radiographically, protrusio acetabuli is present when the medial aspect of the femoral head projects medial to Kohler's (ilioischial) line. This results in medialization of the center of rotation (COR) of the hip. Protrusio acetabuli is typically associated with metabolic bone disease (osteoporosis, osteomalacia, Paget's disease) or inflammatory arthritis (RA or ankylosing spondylitis). Idiopathic acetabular protrusio can occur without the above associated factors however. Patients with protrusio acetabuli typically present with significant restriction of range of motion (ROM) of the hip due to femoral neck and trochanteric impingement in the deep acetabular socket and pain associated with secondary osteoarthritis (OA). Total hip arthroplasty (THA) in patients with protrusion acetabuli is more challenging than THA in patients with a normal hip COR. ROM is typically quite restricted which can compromise surgical exposure. Dislocation of the hip in the patient with a deep socket and medialised COR can be extremely difficult and associated with fracture of the femur if not carefully performed. Restoration of the hip COR to the normal more lateralised position is a principle goal of surgery. This restores more normal mechanics of the hip and has been associated with improved durability. A variety of techniques to accomplish this have been described including medial acetabular bone grafting with cemented cups, protrusio rings or porous coated cementless cups fixed with multiple screws. The latter technique has been shown to be more durable and associated with better outcomes. THA in protrusio acetabuli starts with templating of the preoperative x-rays to determine the optimal acetabular implant size and final position of the acetabular component that restores the hip COR to the normal position. Patients with protrusio acetabuli often have varus oriented femoral necks and the femur needs to be carefully templated as well to insure that an appropriate femoral component is available that will allow for restoration of the patient's anatomy. Cartilage covering the thinned medial wall needs to be carefully removed without disruption of the medial acetabular wall. The acetabulum is then carefully reamed with the goal of obtaining stable peripheral rim support of a cementless socket and at least 50% contact of the implant on good quality host bone. Unlike acetabular preparation in the normal hip, preventing the reamer from “bottoming out” is essential in order to obtain desired rim support and return of the hip COR to the normal lateralised position. When good rim support of the reamer is obtained, a trial component is placed and intraoperative x-ray obtained to confirm fit, position and restoration of hip COR. Limited addition reaming can be performed to obtain desired degree of press fit (1‐2mm) and contact with host bone. Morselised autograft from the femoral head and neck is then packed into the medial defect and reverse reamed. The cementless acetabular component is then impacted into position and fixed with screws. Weight bearing is determined by bone quality, size and containment of the medial defect, amount of contact of the cementless cup with host bone and stability of the acetabular construct. Incorporation of autograft bone in the acetabulum and stable long term fixation occurs reliably if stable initial press-fit fixation of the cementless cup is obtained. Restoration of hip COR to within 7mm of its normal location is associated with better implant survival

Introduction. Current standard cups of metal on metal resurfacing hip arthroplasty (RHA) have no dome holes and it is very difficult for surgeons to confirm full seating of these cups. This sometimes results in gap formation between the cup and acetabular floor. Although the incidence of initial gaps using modular press-fit cups with dome screw holes has been reported to range from 20 to 35%, few studies have reported the incidence of gap formation with monoblock metal cups and its clinical consequences in RHA. The purpose of this study was to investigate retrospectively the incidence of initial gap formation and whether the initial gap influences the clinical results in RHA. Material and Method. RHA was performed on 166 hips of 146 patients using the Birmingham Hip Resurfacing (BHR) (MMT, UK) between 1998 and 2007. Mean age at operation was 48.7 years (range, 19-85 years). Mean duration of follow-up was 6.9 years (2.0-10.6). Acetabular reaming was performed with the use of hemispherical reamers and the reamer size was increased up to an odd number diameter which provided tight rim fit in the antero-posterior direction. The same size hemispherical provisional cup with dome holes and slits was used to check the cavity for complete seating. If the provisional cup could not be seated on the floor, reaming was repeated with the same reamer to remove the rim bump until full seating was achieved. Acetabular cups of 1mm larger diameter were impacted into the acetabulum by a press-fit technique. After press-fit fixation, the stability of the cups was confirmed with a synchronized movement of the pelvis and the cup inserter by applying a gentle torque. Clinical evaluation was performed using WOMAC at the latest follow-up. Radiographic assessments were performed using radiographs immediately after the surgery, at 3 weeks, 3 months, 1 year, and then annually thereafter. We evaluated the height of the gap between the cup and acetabular surface, cup inclination angle, cup migration and the time to gap filling. To investigate the relationship between the magnitude of the gap and the radiographic results, the patients were divided into two groups according to the height of the initial gap; the cases with a gap of less than 3 mm on the initial radiograph were grouped into a small gap group, the cases with a gap of 3mm or more were grouped into a large gap group. We compared the changes in the height of the gap, in the cup inclination angle and the cup migration between the groups. Results. Gaps were identified in 21 of 166 hips (13%) on the postoperative radiograph. The average height of the gaps was 2.4 mm (0.56-4.5mm). Of the 21 hips with gaps, there was no revision during the follow up period. 12 of the 21 hips were classified into the small gap group, 9 of the 21 hips were classified into the large gap group. In the small gap group, there were no changes of cup inclination angle of more than 3 degrees nor was there cup migration of more than 3mm. On the other hand, in the large gap group, 6 of the 9 hips showed reductions of the cup inclination angle of more than 3 degrees with 3mm or more of migration during the initial 3 months (P<0.05). After 3 months, neither progressive angle change nor migration of the cup were observed. All acetabular components were judged to be bone ingrown at the last follow up. Conclusions. The incidence of initial postoperative gaps (13%) of this series is similar to that of modular cementless cups fixed with press-fit technique. There were no serious clinical consequences of the initial gap during the 6.9 year follow up. However, gaps of 3mm or more led to early migration of the acetabular component and change in inclination angle