The inadequacy of standard radiographs in detecting flaws in the cement mantle

Accuracy of a non-invasive CT-based measuring technique for cement penetration depth in human tibial UKA

Background

Aseptic loosening of the tibial component remains a major cause of failure in unicompartmental knee arthroplasty (UKA) and may be related to micro-motion at the cement-bone interface due to insufficient cement penetration depth. Cement penetration is therefore taken as an indicator of solid fixation strength and primary stability. However, its non-invasive clinical assessment remains difficult in vivo as conventional x-ray is prone to distortion and CT-scans (computed tomography) are difficult to assess due to metal artifacts. The purpose of this study was to develop and validate a reliable in vivo measuring technique of cement penetration depth in human tibial UKA.

Methods

In an experimental setting, twelve UKA were implanted in fresh-frozen human cadaver knees using a minimal-invasive medial approach. Cement penetration depth was then measured via 1) virtual 3D-models based on metal artifact reduced CT-scans and 2) histological evaluation of nine serial cross-section cuts through the implant-cement-bone-interface. Subsequently, a concordance analysis between the two measuring techniques was conducted.

Results

The average cement penetration depth was 1) 2.20 mm (SD 0.30 mm) measured on metal artifact reduced CT-scans and 2) 2.21 mm (SD = 0.42) measured on serial cuts (p = 0.956). The mean difference between both techniques was 0.01 mm (SD 0.31 mm) and the Person correlation coefficient was r = 0.686 (p = 0.014). All differences were within the upper and lower limit of agreement. There was no evidence of any significant proportional bias between both techniques (p = 0.182).

Conclusions

CT-based non-invasive measurement of cement penetration depth delivers reliable results in measuring the penetration depth in tibial UKA. Thereby, it enables clinicians and researchers to assess the cement penetration for in vivo diagnostics in the clinical setting as well as in vitro biomechanical research with subsequent application of load to failure on the implant-cement-bone-interface.

Routine Use of Radiostereometric Analysis in Elective Hip and Knee Arthroplasty Patients: Surgical Impact, Safety, and Bead Stability

BACKGROUND

Radiostereometric analysis (RSA) accurately assesses arthroplasty implant performance and is routinely used for research purposes. Because of its efficacy in detecting implant failure in research subjects, we deployed the use of RSA beads for routine surveillance in all elective total hip and total knee arthroplasty patients.

METHODS

We retrospectively compared 143 patients who received RSA beads intraoperatively with 343 patients who did not receive RSA beads. Using RSA images, we assessed bead stability at 6 weeks and 1 year postoperatively.

RESULTS

There were no intraoperative complications in the RSA group and two in the control group. No differences were found between groups for the surgical time and the expected blood loss. At 6 weeks, 99.5% of beads were considered to be stable, with stability increasing to 99.7% of beads at 1 year.

CONCLUSION

The routine use of RSA bead insertion is a safe procedure and may provide the ability to assess implant fixation in clinical practice.

Porosity of neat and composite bone cement mantles

The effect of fiber additions to bone cement on femoral cement mantle porosity was determined. Eighteen porcine femurs were implanted with a cemented prosthesis. Three cement types were used: as-received cement, cement with untreated polyethylene terephthalate fibers, and cement with treated polyethylene terephthalate fibers. Radiographs revealed all cement mantles as grade B, with slight radiolucency at the cement-bone interface. The cement mantles were sectioned at 7 levels, and porosity was measured at each level. All specimens had similar porosities, with an overall mean percentage of porosity of 3.3%+/-2.2% and a mean pore count of 208+/-160 per section. The high pore count and porosity were not visible on the standard clinical radiographs.

Vacuum-mixing cement does not decrease overall porosity in cemented femoral stems

The role of vacuum mixing on the reduction of porosity and on the clinical performance of cemented total hip replacements remains uncertain. We have used paired femoral constructs prepared with either hand-mixed or vacuum-mixed cement in a cadaver model which simulated intra-operative conditions during cementing of the femoral component. After the cement had cured, the distribution of its porosity was determined, as was the strength of the cement-stem and cement-bone interfaces.

The overall fraction of the pore area was similar for both hand-mixed and vacuum-mixed cement (hand 6%; vacuum 5.7%; paired t-test, p = 0.187). The linear pore fractions at the interfaces were also similar for the two techniques. The pore number-density was much higher for the hand-mixed cement (paired t-test, p = 0.0013). The strength of the cement-stem interface was greater with the hand-mixed cement (paired t-test, p = 0.0005), while the strength of the cement-bone interface was not affected by the conditions of mixing (paired t-test, p = 0.275). The reduction in porosity with vacuum mixing did not affect the porosity of the mantle, but the distribution of the porosity can be affected by the technique of mixing used.

Prediction of Charnley femoral stem aseptic loosening by early post-operative radiological features

We describe the association between immediate post-operative radiological appearances and early aseptic failure of total hip replacement. Sixty-three hips were entered into the aseptic failure group and 138 into the control group. Alignment of the femoral stem was not associated with failure (p=0.283). Thickness of the cement mantle was associated with failure in Gruen zones 6 (p=0.040) and 7 (p=0.003). A significant association for the presence of radiolucent lines was found for Gruen zones 3 (p=0.0001) and 5 (p=0.0001). Grade of cementation was associated with failure for Barrack grades C (p=0.001) and D (p=0.001). This study has demonstrated that easily applied radiological criteria can be used to identify 'hip arthroplasties at risk' from the immediate post-operative radiograph.

In vitro analysis of the cement mantle of femoral hip implants: development and validation of a CT-scan based measurement tool

We developed, validated and assessed inter- and intraobserver reliability of a CT-scan based measurement tool to evaluate morphological characteristics of the bone-cement-stem complex of hip implants in cadaver femurs. Two different models were investigated: the stem-cavity model using a double tapered polished femoral-stem that is removed after cement curing and the plastic-replica model using a stereolithographic stem replica that is left in place during CT-scanning. Software was developed to segment and analyze connective CT-images and identify the contours of bone, cement, and stem based on their respective gray values. Volume parameters (whole specimen, cement, stem, air contents of bone and cement), concentricity parameters (distances between centroids of stem and cement, cement and bone, stem and bone), contact surfaces (bone/air and cement/bone) and bone cement mantle thickness parameters were calculated. A three-dimensional protocol was developed to evaluate the minimal mantle thickness out of the CT-plane. The average accuracy for surfaces within CT-images was 7.47 mm2 (1.80%), for bone and cement mantle thickness it was 0.51 mm (9.39%), for distances between centroids it was 0.38 mm (18.5%) and contours: 0.27 mm (2.57%). The intra- and interobserver reliability of air content in bone and cement was sub-optimal (intraclass-correlation coefficient (ICC) as low as 0.54 with an average ICC of 0.85). All other variables were reliable (ICC>0.81, average ICC: 0.96). This in vitro technique can assess characteristics of cement mantles produced by different cementing techniques, stem types or centralizers.

Intra-operative specimen analysis using faxitron microradiography for excision of mammographically suspicious, non-palpable breast lesions

Recent advances in digital imaging have made Faxitron microradiography an attractive alternative to intra-operative conventional specimen radiography (CSR) for the excision of wire-localized breast lesions. Faxitron specimen analysis time, usefulness of digital image manipulation and re-excision rates were evaluated in comparison to CSR in 299 consecutive wire-localized excisions for mammographically suspicious non-palpable breast lesions (172 procedures with Faxitron, 127 with CSR) in a non-randomized study. The corresponding mean operation times were 34.7 vs. 42.7 min and the respective re-excision rates were 19.8% vs. 31.5% (no significant difference on chi analysis P < 0.1). Faxitron digital image manipulation led to cavity biopsies in 50% (60/121) of the cancer excisions. In 19 of these (16%), histological excision margins were converted from incomplete to complete. The shorter Faxitron mean operating time enables an additional wire-localized operation per theatre list. Digital imaging guides the surgeon for additional cavity biopsies, resulting in re-excision rates as good as CSR.

Radiographic evaluation of Charnley cups used in first-time revision: repeated observations for 7-15 years

The radiographs of 46 consecutive polyethylene cups used in cemented first-time revision of the acetabulum were studied up to a mean of 10.7 years (range, 0.5-16.3 years). Six cups developed loosening, of which 2 were revised. New radiolucent lines appeared mainly up to the 3 years follow-up. Progression of radiolucencies to new regions was noted in 28 (61%) cups. This progression occurred as an increase in extension from the periphery to the central region of the interface, whereas the width only showed minor changes. Our findings indicate that even radiolucent lines with a width <1.0 mm should be given attention because these lines can surround the cup before significant migration (>5 mm) is established. True lateral radiographs exposed to visualize the interface add information concerning presence of loosening. According to our opinion, complete radiolucent lines on either the anterior-posterior or lateral view, with a width of 0.3 to 0.5 mm or more, should be regarded as radiographic failure.

The incidence of cement extrusion from the acetabulum in total hip arthroplasty

Acetabular cement extrusion (CE) is a common finding after total hip arthroplasty, but the incidence is unclear from the literature, and there is no accepted way of measuring it. We report a retrospective observational study of 100 patients (25 consecutive patients from each of 4 consultants, 112 hips) to determine the site and incidence of CE on postoperative radiographs, and to measure its area and the maximum distance of the extruded cement from the edge of the cement mantle. We found that 50 of 112 (44.6%) hips showed evidence of CE, with a 15.2% incidence of CE > 200 mm(2).

The John Charnley Award: Three-dimensional analysis of the cement mantle in total hip arthroplasty

Cemented fixation of the femoral stem is the gold standard for patients older than 60 years. The importance of reliably achieving an adequate cement mantle has been shown in many studies. Currently, inspection and grading of plain radiographs is the accepted method for study of the cement mantle. However, the reliability of plain radiographs for this purpose has been questioned. In addition, the interobserver agreement of current grading systems has been shown to be limited. A new in vitro method of cement mantle analysis is described. Plastic replicas of six contemporary stems were implanted into femurs from cadavers. The specimens were imaged with a computed tomography scanner. Detailed, computer-assisted analysis of mantle thickness was done. Comparisons were made between designs. A subset was compared with standard radiographs. Plain radiographs overestimated thickness and underestimated the deficiencies. There was significant variability in the mantle produced by the different designs. Commonly used designs had deficiencies in their mantles by standard criteria despite proper surgical technique. The importance of being fully acquainted with the particular implant one uses is emphasized by these results. This is a valuable technique for investigation of the effects on the cement mantle of implant design, surgical technique, and patient anatomy.

Dependency of cement mantle thickness on femoral stem design and centralizer

Deficient cement mantles may be detrimental with regard to long-term outcome of cemented femoral stems. We performed a cadaver study on 48 left femora with 4 different stem designs (1 anatomic, 3 straight) to study the influence of stem design, centralizer, and femur type on cement mantle thickness. A radiographic and microradiograhic analysis was done. Overall, 88% of stems were aligned within 1 degrees of neutral in the frontal plane. In Gruen zones 1 through 7, we measured 24 thin cement mantles (<2 mm) in 19 specimens with no correlation to stem design or zone. In the sagittal plane, typical areas of thin cement mantles were identified in Gruen zones 8 and 9 (n = 39) and 12 (n = 21). The anatomic stem design carried the lowest risk (54%) of producing a thin cement mantle proximally in Gruen zones 8 and 9. The risk for straight stem designs was >90%. Straight stems without centralizer showed the highest risk of thin cement mantles in Gruen zone 12 (93%). Centralizers were efficient to prevent thin cement mantles in zone 12 but had no effect proximally. Lateral radiographs are essential to allow for adequate radiographic assessment of the cement mantle and stem alignment. There is a high risk of producing thin cement mantles in Gruen zones 8 and 9, in particular when straight stems are used. Posterior canal entry and low neck osteotomies are essential. Anatomic stems respect the anatomy, allow for more even cement mantles, minimize the risk of thin cement mantles without the use of centralizers, and may be considered in the femur with marked proximal bow.