We studied 56 patients with fractures of the tibial shaft in a multicentre prospective randomised trial of three methods of external fixation. Group I was treated with a fixator which was unlocked at 4 to 6 weeks to allow free axial compression (axial dynamisation) with weight-bearing. Group II was treated with a fixator that was similarly unlocked at 4 to 6 weeks but included a small silicone spring which on weight-bearing could be compressed by up to 2 mm. this spring returns to its original length on cessation of weight-bearing thus allowing cycles of motion of up to 2 mm. Group III had a spring fixator like group II, but it was unlocked from the start to allow cyclical micromovement as soon as weight-bearing began. Fracture healing was monitored by the measurement of fracture stiffness. We defined healing as achieving a stiffness of 15 Nm per degree. The mean time was 14.1 weeks in group I, 15.9 weeks in group II, and 19.3 weeks in group III. The difference between groups was statistically significant (p = 0.004). The 95% confidence intervals for the average delay in healing with early cyclical micromovement (group III) as compared with later axial dynamisation (group I) was 1.8 to 8.7 weeks. The healing time in patients whose cyclical micromovement was delayed for 4 to 6 weeks (group II) was between these two extremes, but the differences from either of the other groups could have been due to patient selection. In the patients who completed the full trial, there were pin-track infections in over 60% of those in the cyclical micromovement groups compared with 20% in the axial dynamisation group (p = 0.03).(ABSTRACT TRUNCATED AT 250 WORDS)

Fundamental engineering considerations indicate that micro-movement of the components of any hip arthroplasty is inevitable: stress cannot exist without strain and vice versa. Micromovement can be classified either as inducible recoverable movement that takes place between the weight-bearing and non-weight-bearing phases of each stride, or as non-recoverable displacement between successive loading cycles. Radiostereometric analysis is now sufficiently advanced to clarify migration and its significance, and is beginning to throw light on the extent and significance of recoverable cyclical micromovement. We discuss the value of radiostereometric analysis in identifying, early in their in-service life, implants that are likely to loosen

Radiostereometric analysis (RSA) can detect early micromovement in unstable implant designs which are likely subsequently to have a high failure rate. In 2010, the Articular Surface Replacement (ASR) was withdrawn because of a high failure rate. In 19 ASR femoral components, the mean micromovement over the first two years after implantation was 0.107 mm (. sd. 0.513) laterally, 0.055 mm (. sd. 0.204) distally and 0.150 mm (. sd. 0.413) anteriorly. The mean backward tilt around the x-axis was -0.08° (. sd. 1.088), mean internal rotation was 0.165° (. sd. 0.924) and mean varus tilt 0.238° (. sd. 0.420). The baseline to two-year varus tilt was statistically significant from zero movement, but there was no significant movement from one year onwards. We conclude that the ASR femoral component achieves initial stability and that early migration is not the mode of failure for this resurfacing arthroplasty

Although it has been well established that fracture healing is influenced by the mechanical environment, the optimal parameters have not yet been established. In two groups of sheep an experimental tibial diaphysial fracture was created, and stabilised using external skeletal fixation. In one group rigid fixation was maintained throughout fracture healing; in the other group controlled axial micromovement, with a loading regime known to be osteogenic in intact bones, was applied for a short period daily. A significant improvement in healing was associated with the application of controlled micromovement. Data from these experiments provide the basis for improving the conditions for fracture healing and may assist in the prevention of delayed union

In a prospective study between 2000 and 2005, 22 patients with primary osteoarthritis of the shoulder had a total shoulder arthroplasty with a standard five-pegged glenoid component, 12 with non-offset humeral head and ten with offset humeral head components. Over a period of 24 months the relative movement of the glenoid component with respect to the scapula was measured using radiostereometric analysis.

Nine glenoids needed reaming for erosion. There was a significant increase in rotation about all three axes with time (p < 0.001), the largest occurring about the longitudinal axis (anteversion-retroversion), with mean values of 3.8° and 1.9° for the non-offset and offset humeral head eroded subgroups, respectively. There was also a significant difference in rotation about the anteversion-retroversion axis (p = 0.01) and the varus-valgus (p < 0.001) z-axis between the two groups. The offset humeral head group reached a plateau at early follow-up with rotation about the z-axis, whereas the mean of the non-offset humeral head group at 24 months was three times greater than that of the offset group accounting for the highly significant difference between them.

To measure any observed migration and rotation of humeral and ulnar components using radiostereometric analysis. From 2002–2004 in a prospective ongoing study, twelve elbows in patients treated with either a linked or unlinked Acclaim total elbow prosthesis were included in a radiostereometry study. Six tantalum markers were introduced into the humerus another three markers were located on a humeral component. Four markers were placed in to ulna and three markers located on the ulnar component. RSA radiographs were taken postoperatively, six, twelve and twenty-four months. The radiographs were digitised and analysed using UmRSA software. The relative movement of the humeral and ulnar implants with respect to the bone was measured. At twelve months, the largest segment translation of the humeral component was in the anterior/posterior direction with a mean of 0.44mm followed by medial/lateral translation of 0.39 mm; there was minimal proximal/ distal translation or with a mean of 0.16mm. Paired t-tests between twelve and 24 months segment translation data showed the mean differences to be no more than 0.056mm. The largest rotation at twelve months was anteversion/retroversion with a mean of 2.40deg, anterior tilt had a mean of 1.20deg and varus/valgus tilt was minimal mean 0.60deg. Mean difference between twelve and 24 months segment rotation was no more than 0.30deg. In contrast, humeral tip motion produced a mean of 1.1mm at 12 months dominated by movement in the plane horizontal plane with a mean difference at 24 months of 0.06mm. No patients could be measured for segment micromotion of the ulnar component due to technical difficulty in visualising tantalum markers in the ulna. Early micromotion of the Acclaim humeral implant occurs mostly by rotation about the vertical axis accompanied by anterior tilt. This motion reaches a plateau at 12 months after operation.

Resurfacing of the humeral head is commonly used within the UK to treat osteoarthritis (OA) of the shoulder. We present the results of a small prospective randomised study of this procedure using the Global CAP prosthesis with two different coatings, Porocoat and DuoFix hydroxyapatite (HA). We followed two groups of ten patients with OA of the shoulder for two years after insertion of the prosthesis with tantalum marker beads, recording pain, Constant–Murley and American Shoulder and Elbow Surgeons (ASES) outcome scores, and using radiostereometric analysis to assess migration. The outcomes were similar to those of other series, with significant reductions in pain (p = 0.003) and an improvement in the Constant (p = 0.001) and ASES scores (p = 0.006). The mean migration of the prosthesis three months post-operatively was 0.78 mm (0.51 to 1.69) and 0.72 mm (0.33 to 1.45) for the Porocoat and DuoFix groups, respectively. Analysis of variance indicated that the rate of migration reached a plateau after three months post-operatively in both groups. At follow-up of two years the mean migration was 1 mm (sd 1 (0.25 to 3.32)); in the Porocoat group and 0.8 mm (sd 0.4 (0.27 to 1.45)) in the DuoFix HA group. Significant migration of the prosthesis was seen in one patient who had received an anterior humeral bone graft. This prosthesis was later revised after 2.7 years.

The addition of a coating of HA to the sintered surface does not improve fixation of this prosthesis.

Cite this article: Bone Joint J 2014;96-B:1077–81.

To measure any observed differences in migration and rotation between keeled and pegged glenoid components using roentgen stereo-photogrammetric analysis.

Between 2000–2001 in a prospective randomised study, 20 patients with osteoarthritis had a TSR with roentgen stereo-photogrammetric analysis. Five tantalum markers were introduced into the scapula and acromion, spaced widely apart. Another 4 markers were placed in either a pegged or keeled glenoid component.

RSA radiographs were taken postoperatively, three, six, twelve and eighteen months. The radiographs were digitised and analysed using dedicated software (UmRSA). The relative movement of the glenoid with respect to the scapula was measured.

The largest translation for the keeled components was in the proximal/distal direction with a mean of 1.3mm, the pegged group mean was 0.27mm. This difference was significant, P = 0.001. Both other translation axes showed no significant difference between the two component types. Medial/lateral migration gave means of 0.38mm(keel) and 0.52mm(peg) and the anterior/ posterior translation with means of 0.54mm(keel) and 0.41mm(peg). Maximal total point motion mean values at 18 months were 2.6mm for keeled and 1.1mm for pegged glenoid components. This difference was also significant P=0.001

The largest rotation was anterior/posterior tilt with means of 3.5°(keel) and 1.1°(peg) this difference was significant p=0.005; varus/valgus tilt with a mean of 3.2°(keel) compared to 20(peg) was again, significant P = 0.002. and finally anteversion/retroversion means were 3.3°(keel) and 2.7°(peg). Multivariate analysis identified 2 principal components from the keeled data each accounting for 35% of the variation in the data. The first included transverse and saggital translation together with rotation about the longitudinal axis; the second component consisted of translation on the longitudinal axis with rotation about the transverse axis.

This was a randomised controlled trial studying the safety of a new short metaphyseal fixation (SMF) stem. We hypothesised that it would have similar early clinical results and micromovement to those of a standard-length tapered Synergy metaphyseal fixation stem. Using radiostereometric analysis (RSA) we compared the two stems in 43 patients. A short metaphyseal fixation stem was used in 22 patients and a Synergy stem in 21 patients. No difference was found in the clinical outcomes pre- or post-operatively between groups. RSA showed no significant differences two years post-operatively in mean micromovement between the two stems (except for varus/valgus tilt at p = 0.05) (subsidence 0.94 mm (. sd.  1.71) vs 0.32 mm (. sd. 0.45), p = 0.66; rotation 0.96° (. sd. 1.49) vs 1.41° (. sd. 2.95), p = 0.88; and total migration 1.09 mm (. sd. 1.74) vs 0.73 mm (. sd. 0.72), p = 0.51). A few stems (four SMF and three Synergy) had initial migration > 1.0 mm but stabilised by three to six months, with the exception of one SMF stem which required revision three years post-operatively. For most stems, total micromovement was very low at two years (subsidence < 0.5 mm, rotation < 1.0°, total migration < 0.5 mm), which was consistent with osseous ingrowth. The small sample makes it difficult to confirm the universal applicability of or elucidate the potential contraindications to the use of this particular new design of stem. Cite this article: Bone Joint J 2015; 97-B:595–602

Finite element analysis was used to examine the initial stability after hip resurfacing and the effect of the procedure on the contact mechanics at the articulating surfaces. Models were created with the components positioned anatomically and loaded physiologically through major muscle forces. Total micromovement of less than 10 μm was predicted for the press-fit acetabular components models, much below the 50 μm limit required to encourage osseointegration. Relatively high compressive acetabular and contact stresses were observed in these models. The press-fit procedure showed a moderate influence on the contact mechanics at the bearing surfaces, but produced marked deformation of the acetabular components. No edge contact was predicted for the acetabular components studied. It is concluded that the frictional compressive stresses generated by the 1 mm to 2 mm interference-fit acetabular components, together with the minimal micromovement, would provide adequate stability for the implant, at least in the immediate post-operative situation

We performed a randomised, radiostereometric study comparing two different bone cements, one of which has been sparsely clinically documented. Randomisation of 60 total hip replacements (57 patients) into two groups of 30 was undertaken. All the patients were operated on using a cemented Charnley total hip replacement, the only difference between groups being the bone cement used to secure the femoral component. The two cements used were Palamed G and Palacos R with gentamicin. The patients were followed up with repeated clinical and radiostereometric examinations for two years to assess the micromovement of the femoral component and the clinical outcome. The mean subsidence was 0.18 mm and 0.21 mm, and the mean internal rotation was 1.7° and 2.0° at two years for the Palamed G and Palacos R with gentamicin bone cements, respectively. We found no statistically significant differences between the groups. Micromovement occurred between the femoral component and the cement, while the cement mantle was stable inside the bone. The Harris hip score improved from a mean of 38 points (14 to 54) and 36 (10 to 57) pre-operatively to a mean of 92 (77 to 100) and 91 (63 to 100) at two years in the Palamed G and Palacos R groups, respectively. No differences were found between the groups. Both bone cements provided good initial fixation of the femoral component and good clinical results at two years

After cemented total hip arthroplasty (THA) there may be failure at either the cement-stem or the cement-bone interface. This results from the occurrence of abnormally high shear and compressive stresses within the cement and excessive relative micromovement. We therefore evaluated micromovement and stress at the cement-bone and cement-stem interfaces for a titanium and a chromium-cobalt stem. The behaviour of both implants was similar and no substantial differences were found in the size and distribution of micromovement on either interface with respect to the stiffness of the stem. Micromovement was minimal with a cement mantle 3 to 4 mm thick but then increased with greater thickness of the cement. Abnormally high micromovement occurred when the cement was thinner than 2 mm and the stem was made of titanium. The relative decrease in surface roughness augmented slipping but decreased debonding at the cement-bone interface. Shear stress at this site did not vary significantly for the different coefficients of cement-bone friction while compressive and hoop stresses within the cement increased slightly

Purpose: The causes of glenoid loosening are multifactorial (implant design, surgical technique, bone properties, soft tissue properties). This biomechanical study was conducted to evaluate the consequences of two clinical problems often encountered in shoulder arthroplasty: subscapular tension and glenoid retroversion. Material and methods: We developed a 3D model of the shoulder including the rotator cuff. A total prosthesis was implanted by digital modellisation. The humeral prosthesis imitated the adaptable third-generation implants, with a stem and a portion of a metal sphere, were used to achieve anatomic reconstruction of the proximal humerus. The polyethylene glenoid, cemented to bone, had a central stem and a flat base. Two subscapular tension (normal and twice normal) and two glenoid positions (0° and 20° retroversion) were tested. External rotation (0–40°) and internal rotation (0–60°) were simulated. We calculated displacement of the glenohumeral contact point, joint forces and contact pressures, interosseous glenoid stress, and micromovement of the bone-cement-implant interfaces. Results: Subscapular tension produced increased forces and joint pressures, associated with moderate posterior translation of the glenohumeral contact point. Retroversion induced more marked posterior displacement of the contact point, leading to significantly higher intraosseous glenoid stress and micromovements at the interfaces. The association of subscapular tension and glenoid retroversion produced important concentration of stress forces in the posterior part of the glenoid and increased all the micromovements. Discussion: Subscapular tension and retroversion of the glenoid implant have significant biomechanical effects which can favour glenoid loosening. Correction of these two parameters must be carefully controlled during shoulder arthroplasty

In previous studies, we have demonstrated a fibrocartilaginous membrane around hydroxyapatite-coated implants subjected to micromovement in contrast to the fibrous connective tissue which predominates around similarly loaded titanium alloy implants. In the present study, in mature dogs, we investigated the effect of immobilising titanium (Ti)- or hydroxyapatite (HA)-coated implants already surrounded by a movement-induced fibrous membrane and compared the results with those of similar implants in which continuous micromovement was allowed to continue. The implants were inserted in the medial femoral condyles of 14 dogs and subjected to 150 microns movements during each gait cycle. After four weeks (when a fibrous membrane had developed), half the implants were immobilised to prevent further micromovement. The dogs were killed at 16 weeks and the results were evaluated by push-out tests and histological analysis. The continuously loaded Ti-coated implants were surrounded by a fibrous membrane, whereas bridges of new bone anchored the HA-coated implants. The immobilised implants were surrounded by bone irrespective of the type of coating. Push-out tests of the continuously loaded implants showed better fixation of those with HA coating (p < 0.001). The immobilised Ti-coated implants had four times stronger fixation than did continuously loaded Ti-coated implants (p < 0.01) but there was no equivalent difference between the two groups of HA-coated implants. The amount of bone ingrowth was greater into immobilised HA-coated implants than into immobilised Ti-coated implants (p < 0.01). Two-thirds of the HA coating had been resorbed after 16 weeks of implantation, but 25% of this resorbed HA had been replaced by bone.(ABSTRACT TRUNCATED AT 250 WORDS)

Purpose: We performed mechanical trials to quantify the contribution of locking to the stability of revision femoral implants. The implant tested was a revision prosthesis with anatomic metaphyseal contact locked with three distal bolts measuring 4.5 mm. Material and methods: Twelve implants were impacted into composite saw bones with constant and known dimensions and mechanical properties. Three displacement sensors were used to measure micromovements between the prosthesis and the bone: three specially designed force sensors were inserted into the bolt holes to measure the force distributions for each hole. Measurements were made with an Instron. Compression cycles (780 1-Hz cycles, 100daN applied to the femoral head) and torsion cycles (780 1-Hz cycles, 4.5 Nm applied to the femoral head) were used to simulate loading and weight-bearing and to estimate the evolution of the system. Trials were conducted in two different configurations: stable metaphyseal prosthesis, unstable metaphyseal prosthesis (simulating surgical resection). These two configurations were tested with a locked and with a non-locked implant. Results: Loading distribution between the bolts was variable and depended on the insertion conditions, implant/bolt tolerance, and the quality of the supporting bone. In the “stable” metaphyseal configuration, the bolts carried a large percentage of the compression force (up to 30%) despite the support provided by the metaphysis; when exposed to torsion stress, the metaphyseal form of the prosthesis carried the charge and avoided this phenomenon. Locking had only minimal effect on micromovements, the impaction and the form of the prosthesis maintaining its stability. For the “unstable” configuration, locking created a stable situation: micromovements were limited to those observed in the stable prosthesis (< 150 μm), compatible with bone regrowth. The bolts carried most of the charge (74.8 ±20%; 56.0±41.7%) during the compression and torsion tests. Loading created major stress within the bolts whose properties (strict diameter 4.5 mm, lateral threading) should be taken into consideration to avoid risk of rupture beyond the elastic limit of the material. Discussion: These results can be reasonably extrapolated to surgical situations leading to the following conclusions: locking is useful and reliable after surgical resection, all the holes available should be used for locking, “rational” unlocking can be useful if “physiological” metaphyseal stress is desired

Mechanical loosening which begins with early-onset migration of the prosthesis is the major reason for failure of the Souter-Strathclyde elbow replacement. In a prospective study of 18 Souter-Strathclyde replacements we evaluated the patterns of migration using roentgen stereophotogrammetric analysis. We had previously reported the short-term results after a follow-up of two years which we have now extended to a mean follow-up of 8.2 years (1 to 11.3). Migration was assessed along the co-ordinal axes and overall micromovement was expressed as the maximum total point movement. The alignment of the prosthesis and the presence of radiolucent lines were examined on conventional standardised radiographs. All the humeral components showed increased and variable patterns of migration at the extended follow-up and four humeral components were revised. The maximum total point movement at two years in the revised prostheses was 1.8 mm (. sd. 1.0) and in the non-revised 0.7 mm (. sd. 0.5, p = 0.01). Most humeral components migrated into external rotation resulting in an anterior and varus tilt. The ulnar components remained stable

Diaphyseal fractures of the tibia in 80 patients were treated by external skeletal fixation using a unilateral frame, either in a fixed mode or in a mode which allowed the application of a small amount of predominantly axial micromovement. Patients were allocated to each regime by random selection. Fracture healing was assessed clinically, radiologically and by measurement of the mechanical stiffness of the fracture. Both clinical and mechanical healing were enhanced in the group subjected to micromovement, compared to those treated with frames in a fixed mode possessing an overall stiffness similar to that of others in common clinical use. The differences in healing time were statistically significant and independently related to the treatment method. There was no difference in complication rates between treatment groups

Accurate quantitative measurements of micromovement immediately after operation would be a reliable indicator of the stability of an individual component. We have therefore developed a system for measuring micromovement of the tibial component using three non-contact displacement transducers attached to the tibial cortex during total knee arthroplasty (TKA). Using this system we measured the initial stability in 31 uncemented TKAs. All the tibial components were fixed by a stem and four screws. The initial stability was defined as the amount of displacement when a load of 20 kg was applied. The mean subsidence was 60.7 μm and the mean lift-off was 103.3 μm. We also studied the migration of the tibial component using roentgen stereophotogrammetric analysis (RSA) for up to two years after operation. Most migration occurred during the first six months, after which all prostheses remained stable. We defined migration as the maximum total point motion (MTPM) at two years after operation. The mean migration was 1.29 mm at two years. Our results show that there was a significant correlation between the initial stability and migration (p < 0.05) and emphasise the importance of the initial stability of the tibial component

This was a safety study where the hypothesis was that the newer-design CPCS femoral stem would demonstrate similar early clinical results and micromovement to the well-established Exeter stem. Both are collarless, tapered, polished cemented stems, the only difference being a slight lateral to medial taper with the CPCS stem. A total of 34 patients were enrolled in a single-blinded randomised controlled trial in which 17 patients received a dedicated radiostereometric CPCS stem and 17 a radiostereometric Exeter stem. No difference was found in any of the outcome measures pre-operatively or post-operatively between groups. At two years, the mean subsidence for the CPCS stem was nearly half that seen for the Exeter stem (0.77 mm (−0.943 to 1.77) and 1.25 mm (0.719 to 1.625), respectively; p = 0.032). In contrast, the mean internal rotation of the CPCS stem was approximately twice that of the Exeter (1.61° (−1.07° to 4.33°) and 0.59° (0.97° to 1.64°), respectively; p = 0.048). Other migration patterns were not significantly different between the stems. The subtle differences in designs may explain the different patterns of migration. Comparable migration with the Exeter stem suggests that the CPCS design will perform well in the long term