Additive manufactured push‐fit implant fixation with screw‐strength pull out

Additive manufacturing offers exciting new possibilities for improving long‐term metallic implant fixation in bone through enabling open porous structures for bony ingrowth. The aim of this research was to investigate how the technology could also improve initial fixation, a precursor to successful long‐term fixation. A new barbed fixation mechanism, relying on flexible struts was proposed and manufactured as a push‐fit peg. The technology was optimized using a synthetic bone model and compared with conventional press‐fit peg controls tested over a range of interference fits. Optimum designs, achieving maximum pull‐out force, were subsequently tested in a cadaveric femoral condyle model. The barbed fixation surface provided more than double the pull‐out force for less than a third of the insertion force compared to the best performing conventional press‐fit peg (p < 0.001). Indeed, it provided screw‐strength pull out from a push‐fit device (1,124 ± 146 N). This step change in implant fixation potential offers new capabilities for low profile, minimally invasive implant design, while providing new options to simplify surgery, allowing for one‐piece push‐fit components with high levels of initial stability. © 2017 The Authors. Journal of Orthopaedic Research® Published by Wiley Periodicals, Inc. on behalf of the Orthopaedic Research Society. J Orthop Res 36:1508–1518, 2018.     

Stability of small pegs for cementless implant fixation

Most glenoid implants rely on large centrally located fixation features to avoid perforation of the glenoid vault in its peripheral regions. Upon revision of such components there may not be enough bone left for the reinsertion of an anatomical prosthesis. Multiple press‐fit small pegs would allow for less bone resection and strong anchoring in the stiffer and denser peripheral subchondral bone. This study assessed the fixation characteristics, measured as the push‐in (P _in) and pull‐out (P _out) forces, and spring‐back, measured as the elastic displacement immediately after insertion, for five different small press‐fitted peg configurations manufactured out of UHMWPE cylinders (5 mm diameter and length). A total of 16 specimens for each configuration were tested in two types of solid bone substitute: Hard (40 PCF, 0.64 g/cm³, worst‐case scenario of P _in) and soft (15 PCF, 0.24 g/cm³, worst‐case scenario of spring‐back and P _out). Two different diametric interference‐fits were studied. Geometries with lower stiffness fins (large length to width aspect ratio) were the best performing designs in terms of primary fixation stability. They required the lowest force to fully seat, meaning they are less damaging to the bone during implantation, while providing the highest P _out/P _in ratio, indicating that when implanted they provide the strongest anchoring for the glenoid component. It is highlighted that drilling of chamfered holes could minimize spring‐back displacements. These findings are relevant for the design of implants press‐fitted pegs because primary fixation has been shown to be an important factor in achieving osseointegration and longevity of secondary fixation. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2765–2772, 2017.

     

Hydroxyapatite coating enhances fixation of porous coated implants: A comparison in dogs between press fit and noninterference fit

Intimate contact at the bone-porous surface interface is not always achievable in noncemented prosthetic implantation. We investigated the effect of hydroxyapatite (HA) coating on skeletal attachment in non-interference fit 4 weeks after implantation in 6 mature dogs. The push-out test of HA-coated implants surrounded by a 1-mm gap showed a twofold increased shear strength and fivefold increased shear stiffness compared with titanium alloy (Ti) coated implants. The fixation of Ti implants was reduced by two thirds when inserted in a gap as compared with press fit, whereas HA-coated implants in gap showed anchorage close to implants in press fit. Only minor differences were found between HA and Ti implants in press fit. Histomorphometric analysis showed a significant increase in bone in direct contact to HA-coated implant as compared with Ti implants inserted both in gap and press fit. The study indicates that tightness of surgical fit is an important factor for sufficient fixation of the implant. However, our results demonstrate that hydroxyapatite coating almost eliminates the negative influence of noninterference fit between bone and unloaded implant.     

Hydroxyapatite coating enhances fixation of porous coated implants. A comparison in dogs between press fit and noninterference fit

Intimate contact at the bone-porous surface interface is not always achievable in noncemented prosthetic implantation. We investigated the effect of hydroxyapatite (HA) coating on skeletal attachment in noninterference fit 4 weeks after implantation in 6 mature dogs. The push-out test of HA-coated implants surrounded by a 1-mm gap showed a twofold increased shear strength and fivefold increased shear stiffness compared with titanium alloy (Ti) coated implants. The fixation of Ti implants was reduced by two thirds when inserted in a gap as compared with press fit, whereas HA-coated implants in gap showed anchorage close to implants in press fit. Only minor differences were found between HA and Ti implants in press fit. Histomorphometric analysis showed a significant increase in bone in direct contact to HA-coated implant as compared with Ti implants inserted both in gap and press fit. The study indicates that tightness of surgical fit is an important factor for sufficient fixation of the implant. However, our results demonstrate that hydroxyapatite coating almost eliminates the negative influence of noninterference fit between bone and unloaded implant.     

Effect of cavity preparation and bone mineral density on bone‐interface densification and bone‐implant contact during press‐fit implantation of hip stems

Implant loosening and periprosthetic fracture are two major revision causes for uncemented hip stems. The chosen method of cavity preparation could play a key role for both failure mechanisms. The aim of this study was to determine the dependence of the broach type as well as patient bone mineral density (BMD) on densification and contact conditions at the bone‐implant interface. Hip stems were implanted into cadaveric femora using compaction, blunt extraction or sharp extraction broaches with computed tomography scans performed prior to broaching, after broaching and after stem implantation. Proximal periprosthetic bone densification as well as press‐fit, contact area and stem seating relative to the last broach were determined. Median bone densification was higher with the compaction and blunt extraction broaches compared to sharp extraction broaches (181% and 177%, respectively, p = 0.002). The bone densification of femora prepared with compaction broaching increased with higher BMD (R² = 0.183, p = 0.037), while stem seating decreased with higher BMD for all broach types (R² = 0.259, p = 0.001). Incomplete seated prostheses were associated with smaller press‐fit and bone‐implant contact area (R² = 0.249, p = 0.001; R² = 0.287, p < 0.001). Clinical Significance: The results suggest that compaction broaching maximizes bone densification in patients with higher bone density. However, there appears to be an increased risk of insufficient stem seating in high‐density bone that could limit the benefits for primary stability. For lower quality bone, the broach type appears to play a lesser role, but care must be taken to limit extensive stem seating which might increase periprosthetic fracture risk. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1580–1589, 2019.     

The relevance of hydroxyapatite and spongious titanium coatings in fixation of cementless stems

Pure titanium rods plasma-spray coated with hydroxyapatite (HA) or porous titanium (Ti) of controlled roughness were implanted bilaterally in the distal femur of Sprague-Dawley rats to compare the extent of bone growth on the two types of coating. The relevance of other factors, like mechanical stability and biological adaptation of the bone to the insertion of a foreign body implant, were investigated in femora which were over-reamed (absence of primary fit) or reamed without insertion of the rod. Continuous tetracycline labeling for the first 30 days and for the last 2 weeks in the 90-day group was performed; histological/histometric, fluorescence and microangiographic studies were carried out on serial sections of the implanted and control femora. In the group of stable implants, HA-coated rods showed 90% integration versus 53% with Ti-coated implants (P < 0.001); in over-reamed implants neither surface bone growth nor endosteal fixation occurred, and both types of rods were surrounded by a thick layer of connective tissue. The study documented early adhesion of osteoblasts and direct deposition of bone matrix on the substrate, while on spongious titanium osteogenesis was observed only in proximity to the surface. Remodeling of the reactive, primary bone to mature, lamellar bone took the form of a capsule surrounding the implants and radial bridges connecting the latter to the endosteal surface. The number, height and thickness of these bridges appeared to be the factors determining implant stability, rather than the extent of the bony capsule on the perimeter of the implant. Integration was a function not only of mechanical conditions and surface geometry, but also of the biological response of the whole bone to changes in the vascularization pattern. The reported phenomena can be seen more easily in experimental models involving small rodents because of their fast bone turnover and revascularization, but it is expected that they take place, even at a lower speed, in clinical situations like cementless stems of total hip replacement. Received: 22 August 1995     

Comparative in vitro assessment of the primary stability of cementless press-fit acetabular cups

Long-term stable osseointegration of porous-coated acetabular cups depends on bony ingrowth within their porous surface. For ingrowth to take place, one must ensure rigid initial fixation of the implant, by means of screws or by impaction or using a threaded ring. Primary stability is a prerequisite for long term stability through bony ingrowth. We tested several cups commonly used in our department to assess their primary stability. The study was done using synthetic EP-Dur polyurethane resin blocks (Bayer, Leverkusen, Germany). The blocks were fixed at a 45 degrees angle to the horizontal. They were subsequently reamed using the appropriate reamers and the cups tested were impacted into the resin blocks. Eleven 52-mm cups were tested. The pull out force necessary to extract each cup was measured. The pull-out strength ranged from 7.63 to 55.46 Nm. We noted that the closer the cup was to a hemisphere, the better was the initial stability. The contact zone was at the periphery, and the greater the contact was with the resin, the better was the stability. Micromovements exceeding 150 microns prevent any bony ingrowth in vivo. Solid osseointegration can thus only be achieved if movements between implant and bone can be prevented. Our study indicated that initial fixation is essentially peripheral and that those cups that demonstrated the highest pull-out values also had the best peripheral contact. Our observations suggest that the geometry of the cup is more important than its surface macrostructure in terms of primary stability. To achieve stable fixation, we recommend using an oversized cup with a flattened dome to allow maximum peripheral contact.     

Impaction bone grafting with freeze-dried irradiated bone. Part I. Femoral implant stabilityCadaver experiments in a hip simulator

Processed freeze-dried irradiated allografts seem to be used less than instead of fresh-frozen allografts for impaction bone grafting in revision hip arthroplasties. Although biologically acceptable, their use is discouraged because of their questionable mechanical properties following freeze-drying and irradiation procedures. To address this question, we impacted freeze-dried grafts in 6 cadaveric femurs and loaded with a cemented Charnley prosthesis. The routinely used fresh-frozen allografts were used as controls in the contralateral side. These constructs were compared simultaneously in a walking hip simulator for their stability during 900,000 loading cycles. The mechanical parameters were axial inducible displacement and subsidence of the implant. The former parameter was lower in the implant mounted on freeze-dried impacted grafts than that mounted on the fresh-frozen bone. The latter parameter was also lower in the freeze-dried group. At the end of the test, we found no implant loosening in either group and their pull out resulted in cement-prosthesis debonding, which showed the mechanical integrity of the impacted grafts. Freeze-dried grafts provide more stable fixation of the stem than fresh-frozen morselized grafts, when tested in a hip simulator.     

Defining the impaction frequency and threshold force required for femoral impaction grafting in revision hip arthroplasty

Background and purpose The two most common complications of femoral impaction bone grafting are femoral fracture and massive implant subsidence. We investigated fracture forces and implant subsidence rates in embalmed human femurs undergoing impaction grafting. The study consisted of two arms, the first examining the force at which femoral fracture occurs in the embalmed human femur, and the second examining whether significant graft implant/subsidence occurs following impaction at a set force at two different impaction frequencies.

Methods Using a standardized impaction grafting technique with modifications, an initial group of 17 femurs underwent complete destructive impaction testing, allowing sequentially increased, controlled impaction forces to be applied until femoral fracture occurred. A second group of 8 femurs underwent impaction bone grafting at constant force, at an impaction frequency of 1 Hz or 10 Hz. An Exeter stem was cemented into the neomedullary canals. These constructs underwent subsidence testing simulating the first 2 months of postoperative weight bearing.

Results No femurs fractured below an impaction force of 0.5 kN. 15/17 of the femurs fractured at or above 1.6 kN of applied force. In the second group of 8 femurs, all of which underwent femoral impaction grafting at 1.6 kN, there was no correlation between implant subsidence and frequency of impaction. Average subsidence was 3.2 (1–9) mm.

Interpretation It is possible to calculate a force below which no fracture occurs in the embalmed human femur undergoing impaction grafting. Higher impaction frequency at constant force did not reduce rates of implant subsidence in this experiment.     

Graft reconstruction of the interosseous membrane in conjunction with metallic radial head replacement: a cadaveric study

PURPOSE: Longitudinal radioulnar dissociation (Essex-Lopresti injury) occurs when traumatic axial loading through the wrist disrupts the interosseous membrane (IOM) of the forearm and fractures the radial head. Proximal migration of the radius results in an ulnar-positive wrist, which can lead to painful ulnar-sided wrist degeneration and distal radioulnar joint instability. The purpose of this study was to measure the ability of an IOM reconstruction used in combination with a metal prosthetic radial head implant to reduce distal ulnar forces in a cadaveric model. The effects of varying the initial graft pretension on distal ulnar force were also studied. METHODS: Twelve fresh frozen and thawed cadaveric forearms had a miniature load cell installed to record force in the distal ulna as the wrist was loaded axially to 134 N of compression force in neutral rotation. Intact forearms were tested first with the elbow in valgus and varus alignments. Loading tests were repeated after (1) insertion of a metal radial head implant that restored radius anatomic length, (2) excision of the IOM (with a radial head implant), and (3) reconstruction of the IOM using a palmaris longus tendon autograft (with a radial head implant). The implant then was removed and loading tests were repeated using 3 levels of initial graft pretension. RESULTS: Mean distal ulnar forces with an intact forearm were 23% of applied wrist force in the varus alignment and 12% in the valgus alignment. Mean force levels after insertion of the implant were 18% (varus) and 13% (valgus); these were not significantly different from corresponding values for the intact forearm. Mean force levels after section of the IOM were 30% (varus) and 14% (valgus); these were not significantly different from corresponding values for the intact forearm (varus and valgus) but the mean for varus was significantly greater than the corresponding value with an implant. After IOM reconstruction with a palmaris longus tendon tensioned to 22 N mean distal ulnar forces were 8% (varus) and 7% (valgus); these means were significantly less than the corresponding values for all prior test conditions. With the radial head removed increasing the level of graft pretension reduced significantly mean distal ulnar force. CONCLUSIONS: With the IOM resected insertion of a metal radial head implant alone did not reduce distal ulnar forces to intact forearm levels. When an IOM reconstruction was performed in combination with the implant mean distal ulnar force was reduced significantly to a level below that for the intact forearm. Applying pretension to the graft displaced the radius distally thereby making the wrist more ulnar negative and reducing distal ulnar force. Our results suggest that an IOM reconstruction used in combination with a metal radial head implant theoretically could help reduce distal ulnar impaction in an Essex-Lopresti injury.     

Shear and tensile strength of hydroxyapatite coating under loading conditions

The shear and tensile strength of a hydroxyapatite (HA) coating on a femoral component was studied after physiological loading conditions in 8 German Shepherds. A proximal macrostructure on the stem was used to protect this region from shear stresses. Another four implantations with uncoated components were used as controls. In vitro testing of the HA layer demonstrated excellent tensile strength and stability to surface deformation. The loaded implants were tested at 6, 12, and 24 weeks. At 6 weeks the HA-coated components could easily be removed by axial loading, whereas the HA layer remained undamaged on the metal. However, pull out tests of implants older than 12 weeks showed complete debonding of the HA layer from the non-macrostructured surface due to shear forces in all cases. Debonding of the HA layer was also observed with microradiography. The macrostructured surface prevented dislodging of the component from this area at pull out test by distributing shear forces. Unlike in uncoated implants, considerable amounts of bone remained attached onto the HA macrostructure when the surrounding femur was pulled out. Shear forces cause debonding of the HA layer, while tensile stress affects failure within the bone. Physiological loading partially produces gaps at the interface so direct transmission of tensile forces onto the bone is lost, and the coating-metal interface becomes the weak point in the system.     

Defining the impaction frequency and threshold force required for femoral impaction grafting in revision hip arthroplasty. A human cadaveric mechanical study

Background and purpose

The two most common complications of femoral impaction bone grafting are femoral fracture and massive implant subsidence. We investigated fracture forces and implant subsidence rates in embalmed human femurs undergoing impaction grafting. The study consisted of two arms, the first examining the force at which femoral fracture occurs in the embalmed human femur, and the second examining whether significant graft implant/subsidence occurs following impaction at a set force at two different impaction frequencies.

Methods

Using a standardized impaction grafting technique with modifications, an initial group of 17 femurs underwent complete destructive impaction testing, allowing sequentially increased, controlled impaction forces to be applied until femoral fracture occurred. A second group of 8 femurs underwent impaction bone grafting at constant force, at an impaction frequency of 1 Hz or 10 Hz. An Exeter stem was cemented into the neomedullary canals. These constructs underwent subsidence testing simulating the first 2 months of postoperative weight bearing.

Results

No femurs fractured below an impaction force of 0.5 kN. 15/17 of the femurs fractured at or above 1.6 kN of applied force. In the second group of 8 femurs, all of which underwent femoral impaction grafting at 1.6 kN, there was no correlation between implant subsidence and frequency of impaction. Average subsidence was 3.2 (1–9) mm.

Interpretation

It is possible to calculate a force below which no fracture occurs in the embalmed human femur undergoing impaction grafting. Higher impaction frequency at constant force did not reduce rates of implant subsidence in this experiment.With the increasing success of acetabular impaction bone grafting in dealing with loss of acetabular bone stock, attempts were made to use the technique to fill femoral bone defects in revision hip arthroplasty. The results of femoral impaction grafting have been satisfactory. In a recent review of over 1,000 femoral impaction graftings, Ornstein et al. (2009) reported 15-year implant survival rates of 94%, with minimal difference in implant survivorship between low- and high-volume units, suggesting that the technique of femoral impaction grafting appears to be, “reliable, can be learned rapidly, and produces a predictably low incidence of aseptic loosening.”Despite these results, however, there is still concern regarding the high rate of complications of femoral impaction grafting, the main complications being operative or perioperative femoral fracture and implant subsidence. Fracture rates of up to 16% have been reported (Masterson et al. 1997, Leopold et al. 1999, Pekkarinen et al. 2000). High rates of implant subsidence have also been described (Eldridge et al. 1997, Masterson et al. 1997), which may be due to inadequate impaction of the morselized bone graft. Thus, increasing the impaction force will ensure improved graft stability but will increase the risk of femoral fracture.In a preliminary study on sow femurs, Flannery et al. (2010) achieved a stable construct without fracture. A stable construct was defined as a femur that underwent impaction bone grafting at sub-threshold force, with a cemented Exeter stem that did not undergo massive early subsidence (10 mm) on initial subsidence testing simulating the first 2 postoperative months of weight bearing. The authors were unable to find any correlation between threshold force, bone mineral density, cortex-to-canal ratio, or cortical thickness in impaction bone grafting in the adult sow femur.In this study, we applied the experimental protocol of Flannery et al. (2010) to a sample of adult human femurs and investigated the above associations—but with the addition of measurement of cortex-to-canal ratios on standardized pretesting plain radiographs.The compaction of the bone graft may also be dependent on the frequency of impaction. In a laboratory study on impacted pig bone, Marck Van Liefland (2006) reported that “high-frequency impaction achieved high compaction at low load.” in a pot of morselized pig bone graft. While high compaction was achieved, the author did not state how far the graft had compressed. “Compared to traditional impaction, the same amount of compaction was achieved at 10–20% of the load. Inversely, compaction almost doubled at the same load.” Thus, prevention of massive early subsidence of the femoral component may also be dependent on the frequency of impaction to obtain adequate compaction, possibly permitting lower forces to be applied at higher frequencies and possibly reducing fracture risk.A final subset of femurs then underwent impaction bone grafting at set impaction force. This was followed by subsidence testing with a cemented Exeter stem, half at an impaction frequency of 1 Hz and the other half at an impaction frequency of 10 Hz.     

Evaluation of pull-off strength and seating displacement of sleeved ceramic revision heads in modular hip arthroplasty

Corrosion in revision total hip arthroplasty can be mitigated using a ceramic head on a well-fixed in situ stem, but concerns of their early failure because of any surface defects on in situ stem necessitates the use of a titanium sleeve, which furnishes a factory-finish surface. These sleeves are manufactured in different sizes allowing neck-length adjustment. The strength of the taper junction of non-sleeved primary heads is well-investigated, but the influence of an interposed titanium sleeve on achieving a secure taper lock is unclear. Therefore, this study aimed to investigate the pull-off strength and seating displacement of revision ceramic heads and titanium taper sleeves. Two different head diameters and two different taper adapter sleeve offset lengths were mated with trunnions at two different impaction forces. The seating displacement and pull-off force was recorded for each specimen. Profilometry of the grooved outer surfaces of the sleeve and trunnion was done before and after testing to analyze the change in surface roughness. The influence of head diameter, sleeve offset, and impaction force on seating displacement and pull-off force was analyzed using analysis of covariance. Pull-off forces for 6 kN assembly force were approximately three times those for 2 kN. The head diameter did not have a significant effect on the measured parameters. Compared with short offset length sleeves, extra-long increased seating displacement by 31% and reduced pull-off forces by 15%. While sleeves of different offset lengths permit control of neck length, surgeons must be careful of the impact of this choice on the stability of implant. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:1523–1528, 2020     

松质骨螺钉骨水泥强化的生物力学和组织学研究

目的通过动物实验观察不同骨水泥强化松质骨螺钉的生物力学和组织学变化动态,为骨质疏松骨折患者内固定提供理论基础。方法在10只杂种犬胫骨近端制作松质骨螺钉植入的动物模型,分别采用碳酸化羟基磷灰石水泥(CHC)和聚甲基丙烯酸甲酯(PMMA)强化,分别于术后5d、4、8、12和16周处死动物,观察螺钉拔出的生物力学和组织学变化。结果CHC强化的螺钉拔出力随手术后时间的延长而逐渐升高,16周时达到(512.5 14.5)N,而PMMA强化组螺钉拔出力则随手术后时间的延长而逐渐降低。CHC-骨界面结合紧密,并且随时间延长出现CHC降解和骨长入,而PMMA-骨界面形成一层纤维组织。结论CHC强化能够提高松质骨螺钉植入体内的稳定性,并且随植入时间延长而逐渐升高。     

膝关节翻修胫骨侧骨缺损打压植骨术后初始稳定性的研究

目的研究膝关节翻修胫骨侧骨缺损采用两种假体打压植骨术后的初始稳定性。方法取5具尸体的胫骨10根,制造T3型(AORI分型)的包容性骨缺损,随机选择同一尸体的左右侧胫骨进行长短柄两种假体的打压植骨翻修,观察松质骨骨密度的变化,进行生物力学测试。结果长短柄假体在1000N循环载荷下的微动没有统计学差别,假体的微动与假体下方移植骨密度呈负线性相关。结论打压植骨技术可以用于胫骨侧严重骨缺损的治疗,在完整皮质骨支撑、骨水泥固定的前提下,长短柄两种假体都能够达到足够的初始稳定。     

Änderungen der stationären Versorgung

Fractures of the radial head are typically due to axial force. The treatment algorithm takes additional ligament injuries into account and demands an individual approach between reconstruction, resection and arthroplasty of the radial head. In case of a stable ulnar collateral ligament, radial head resection is the therapy of choice. Remaining valgus instability is an indication for radial head arthroplasty to restore axial and valgus stability and thus prevent proximal radial migration. Radial head arthroplasty is contraindicated in cases where radial head-preserving therapy might be possible. Therefore, in young patients an attempt of reconstruction is indicated even in comminuted fractures. Existing local or systemic inflammation excludes arthroplasty. Typical complications of radial head arthroplasty include cartilage loss at the capitulum, limited range of motion in the elbow joint and heterotopic ossifications. Furthermore, bone lysis at the shaft has to be expected.     

Principles of fixation of the cementless modular revision stem Revitan

AIM AND METHOD: To analyse the femoral fixation of a modular cementless revision endoprosthesis, eight prosthetic combinations of the Revitan-System (Centerpulse, Winterthur, Switzerland) were implanted in four cadavers. On three cadavers a curved revision Revitan stem was implanted by an endofemoral approach on one side and by a transfemoral approach on the contralateral side. On the fourth cadaver a straight Revitan stem was implanted on one side and a curved Revitan stem on the contralateral side using a transfemoral approach. Transversal slides of 7-8 mm thickness were performed at the fixation areas of each implant and the implant-bone contact was analysed macroscopically and using contact radiography.RESULTS: The straight stem implanted by a transfemoral approach showed a double-conical press-fit fixation with cutting of the eight longitudinal fins into the cortical bone. The curved revision stems implanted by the same approach had a circular surface fixation similar to the press-fit fixation of the straight stem. In contrast, the curved stems implanted by the endofemoral approach (without a window) showed a three-surface fixation. Hereby the two distal fixation areas led to the primary implant stability by three of the four double edges of the octagonal cross-sectional area cutting into the cortical bone. At the proximal fixation zone the implant only had contact of two implant double edges to the cortical bone.CONCLUSION: Different approaches for implantation lead to different fixation techniques of a curved revision stem. This should be considered by analysing postoperative sintering rates of cementless revision stems.     

High tibial osteotomy--primary stability of several implants]

INTRODUCTION: High tibial osteotomy in varus knee has been performed for a long time. Several newer operation techniques have been established in recent years. We tested the primary stability of several of these techniques in vitro. MATERIAL AND METHODS: 10 human cadaveric fresh-frozen specimens were tested with a mean age of 61 years (range 50-72 years), and weight of 65 to 78 kg. The following implants were tested: One-third-tubular plate with cortical screw (AO, Synthes), blade plate with screws (Giebel's plate, Link), bone staples (osteotomy staples, Krackow staples, Smith & Nephew), external fixateur (Orthofix). The specimens were fixed in metal cylinders and then loaded in two different apparati: Shear forces were applied to the osteotomy site by hanging weights parallel to the osteotomy plane in a static-loading frame, and axial forces were applied by a materials testing machine (Zwick). Load displacement was recorded by inductive displacement transducers. RESULTS: The highest stability was achieved by the external fixateurs and the bone staples. Giebel's plate and the one third tubular plate were less stable. Receipt of the medial corticalis was decisive for primary stability of the implants. CONCLUSION: The clinical significance of the results is limited by the relevance of the protocol, which for practical reasons did not account for the soft tissue situation around the knee. Thus, primary stability of the tested devices was generally comparable as long as they were correctly implanted. It was found, that lateral distance of the osteotomized bone should not exceed 3 mm. If the medial cortical is sawed, another medial implant is necessary to ensure sufficient primary stability.     

Postpreparation peri-implant humeral bone density and fixation strength vary based on design in stemless reverse shoulder arthroplasty

Background and HypothesisMorphologic analysis of the proximal humerus has been performed to provide information about regions of best bone quality for stemless implant fixation. However, implant design-related bone analysis regarding primary fixation strength has not been reported. The purpose of this study was to investigate bone volume fraction (BV/TV) and bone mineral density (BMD) in the spatial vicinity of humeral implant fixation and biomechanical performance following placement of two different stemless RSA designs. It was hypothesized that peripheral expandable implant fixation (PEF) would result in residual peri-implant bone with higher BMD and BV/TV and higher fixation strength compared to a central-hexagonal impaction fixation (CHIF) design.MethodsOverall 25 human cadavers were prepped for stemless RSA using either a CHIF (n = 5) or PEF design (n = 20). Micro computed tomography scans were obtained post-preparation to analyze overall BV/TV and BMD in the anchoring region of 2 stemless implant designs. Bone volumes of paired specimens (each group n = 5) were further segmented and divided into three even planar subsections along the implantation direction with four additional radial sectors, each according to their anterior-posterior and medial-lateral direction. Following implantation, biomechanical testing was performed to evaluate ultimate failure load and stiffness of the 2 implants.ResultsPostpreparation paired humeral bone analysis revealed significantly greater residual peri-implant BMD (P< .026) and higher BV/TV for PEF. Overall linear correlation between residual BMD and BV/TV showed good coefficients of determination (R² > 0.69) with significantly higher bone density for PEF (P= .003). Further division in proximal to distal planar subsections of paired specimens showed overall statistically higher BMD and BV/ TV for PEF (at least P< .002). Except from the posterior sector, BMD and BV/TV of overall and specific radial sectors within planar subvolumes were significantly higher for PEF. Biomechanical testing revealed increased ultimate loads (−818 ± 282 N vs. −535 ± 144 N; P= .081) and stiffness (898 ± 106 N/mm vs. 431 ± 121 N/mm; P< .001) with smaller tilting angles at failure for PEF compared to CHIF. A linear relationship between normalized failure load by the implant size and combined bone parameter (BV/TV*BMD) provided an accuracy in the order of R² > 0.89 with significantly higher primary stability for PEF (P = .046).ConclusionImplant design in stemless RSA affects residual bone quality in the anchoring region and primary fixation strength. Peripheral implant anchoring along the insertion direction provides significantly higher residual peri-implant BMD and BV/TV to achieve improved primary fixation with a radial expandable anchoring implant compared to a central-hexagonal impacted design.Level of EvidenceBasic Science Study; Anatomy Study; Imaging; Biomechanics     

Interfacial shear strength of bioactive‐coated carbon fiber reinforced polyetheretherketone after in vivo implantation

Despite the excellent osseointegration of carbon‐fiber‐reinforced polyetheretherketone (CFR/PEEK) with a surface hydroxyapatite (HA) coating, the bone‐implant interfacial shear strength of HA‐coated CFR/PEEK after osseointegration is unclear. We examined the interfacial shear strength of HA‐coated CFR/PEEK implants after in vivo implantation in a rabbit femur‐implant pull‐out test model. HA coating was performed by a newly developed method. Uncoated CFR/PEEK, HA‐coated blasted titanium alloy, and uncoated blasted titanium alloy were used as control implants. The implants were inserted into drilled femoral cortex, and pull‐out tests were conducted after 6 and 12 weeks of implantation to determine maximum interfacial shear strength. The HA‐coated CFR/PEEK (15.7 ± 4.5 MPa) and HA‐coated titanium alloy (14.1 ± 6.0 MPa) exhibited significantly larger interfacial shear strengths than the uncoated CFR/PEEK (7.7 ± 1.8 MPa) and the uncoated titanium alloy (7.8 ± 2.1 MPa) at 6 weeks. At 12 weeks, only the uncoated CFR/PEEK (8.3 ± 3.0 MPa) exhibited a significantly smaller interfacial shear strength, as compared to the HA‐coated CFR/PEEK (17.4 ± 3.6 MPa), HA‐coated titanium alloy (14.2 ± 4.8 MPa), and uncoated titanium alloy (15.0 ± 2.6 MPa). Surface analysis of the removed implants revealed detachment of the HA layer in both the HA‐coated CFR/PEEK and titanium alloy implants. The proposed novel HA coating method of CFR/PEEK significantly increased interfacial shear strength between bone and CFR/PEEK. The achieved interfacial shear strength of the HA‐coated CFR/PEEK implant is of the same level as that of grit‐blasted titanium alloy with HA. © 2012 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 30:1618–1625, 2012