Percutaneous bone cement refixation of aseptically loose hip prostheses: the effect of interface tissue removal on injected cement volumes.

OBJECTIVE: To quantify whether injected cement volumes differed between two groups of patients who underwent experimental minimally invasive percutaneous cement injection procedures to stabilize aseptically loose hip prostheses. One patient group was preoperatively treated using gene-directed enzyme prodrug therapy to remove fibrous interface tissue, while the other group received no preoperative treatment. It was hypothesized that cement penetration may have been inhibited by the presence of fibrous interface tissue in periprosthetic lesions. MATERIALS AND METHODS: We analyzed 17 patients (14 female, 3 male, ages 72-91, ASA categories 2-4) who were treated at our institution. Osteolytic lesions and injected cement were manually delineated using 3D CT image segmentation, and the deposition of injected cement was quantified. RESULTS: Patients who underwent preoperative gene-directed enzyme therapy to remove fibrous tissue exhibited larger injected cement volumes than those who did not. The observed median increase in injected cement volume was 6.8 ml. Higher cement leakage volumes were also observed for this group. CONCLUSION: We conclude that prior removal of periprosthetic fibrous interface tissue may enable better cement flow and penetration. This might lead to better refixation of aseptically loosened prostheses.

Measuring femoral lesions despite CT metal artefacts: a cadaveric study.

OBJECTIVE: Computed tomography is the modality of choice for measuring osteolysis but suffers from metal-induced artefacts obscuring periprosthetic tissues. Previous papers on metal artefact reduction (MAR) show qualitative improvements, but their algorithms have not found acceptance for clinical applications. We investigated to what extent metal artefacts interfere with the segmentation of lesions adjacent to a metal femoral implant and whether metal artefact reduction improves the manual segmentation of such lesions. MATERIALS AND METHODS: We manually created 27 periprosthetic lesions in 10 human cadaver femora. We filled the lesions with a fibrotic interface tissue substitute. Each femur was fitted with a polished tapered cobalt-chrome prosthesis and imaged twice--once with the metal, and once with a substitute resin prosthesis inserted. Metal-affected CTs were processed using standard back-projection as well as projection interpolation (PI) MAR. Two experienced users segmented all lesions and compared segmentation accuracy. RESULTS: We achieved accurate delineation of periprosthetic lesions in the metal-free images. The presence of a metal implant led us to underestimate lesion volume and introduced geometrical errors in segmentation boundaries. Although PI MAR reduced streak artefacts, it led to greater underestimation of lesion volume and greater geometrical errors than without its application. CONCLUSION: CT metal artefacts impair image segmentation. PI MAR can improve subjective image appearance but causes loss of detail and lower image contrast adjacent to prostheses. Our experiments showed that PI MAR is counterproductive for manual segmentation of periprosthetic lesions and should be used with care.

A fluoroscopy-based planning and guidance software tool for minimally invasive hip refixation by cement injection.

PURPOSE: In orthopaedics, minimally invasive injection of bone cement is an established technique. We present HipRFX, a software tool for planning and guiding a cement injection procedure for stabilizing a loosening hip prosthesis. HipRFX works by analysing a pre-operative CT and intraoperative C-arm fluoroscopic images. METHODS: HipRFX simulates the intraoperative fluoroscopic views that a surgeon would see on a display panel. Structures are rendered by modelling their X-ray attenuation. These are then compared to actual fluoroscopic images which allow cement volumes to be estimated. Five human cadaver legs were used to validate the software in conjunction with real percutaneous cement injection into artificially created periprothetic lesions. RESULTS: Based on intraoperatively obtained fluoroscopic images, our software was able to estimate the cement volume that reached the pre-operatively planned targets. The actual median target lesion volume was 3.58 ml (range 3.17-4.64 ml). The median error in computed cement filling, as a percentage of target volume, was 5.3% (range 2.2-14.8%). Cement filling was between 17.6 and 55.4% (median 51.8%). CONCLUSIONS: As a proof of concept, HipRFX was capable of simulating intraoperative fluoroscopic C-arm images. Furthermore, it provided estimates of the fraction of injected cement deposited at its intended target location, as opposed to cement that leaked away. This level of knowledge is usually unavailable to the surgeon viewing a fluoroscopic image and may aid in evaluating the success of a percutaneous cement injection intervention.

Voxel classification and graph cuts for automated segmentation of pathological periprosthetic hip anatomy.

PURPOSE: Automated patient-specific image-based segmentation of tissues surrounding aseptically loose hip prostheses is desired. For this we present an automated segmentation pipeline that labels periprosthetic tissues in computed tomography (CT). The intended application of this pipeline is in pre-operative planning. METHODS: Individual voxels were classified based on a set of automatically extracted image features. Minimum-cost graph cuts were computed on the classification results. The graph-cut step enabled us to enforce geometrical containment constraints, such as cortical bone sheathing the femur's interior. The solution's novelty lies in the combination of voxel classification with multilabel graph cuts and in the way label costs were defined to enforce containment constraints. RESULTS: The segmentation pipeline was tested on a set of twelve manually segmented clinical CT volumes. The distribution of healthy tissue and bone cement was automatically determined with sensitivities greater than 82% and pathological fibrous interface tissue with a sensitivity exceeding 73%. Specificity exceeded 96% for all tissues. CONCLUSIONS: The addition of a graph-cut step improved segmentation compared to voxel classification alone. The pipeline described in this paper represents a practical approach to segmenting multitissue regions from CT.

Comparison of Ho:YAG laser and coblation for interface tissue removal in minimally invasive hip refixation procedures.

Aseptic loosening is the major failure mode for hip prostheses. Currently, loosened prostheses are revised during open surgery. Because of a high complication rate, this demanding procedure cannot be performed in patients with a poor general health. We are developing an alternative minimally invasive refixation procedure that leaves the prostheses in place, but relies on removing the interface membrane and replacing it with bone cement. The aim of this study was to evaluate two interface tissue removal techniques - Ho:YAG laser and coblation - based on two criteria: thermal damage and the ablation rate. In vitro a loosened hip prosthesis was simulated by implanting a prosthesis in each of 10 cadaver femora. Artificially created peri-prosthetic lesions were filled with chicken liver as an interface tissue substitute. We measured temperatures in vitro at different radial distances from the site of removal. Temperatures during removal were recorded both inside the interface tissue and in the surrounding bone. This study demonstrated that temperatures generated in the bone do not result in thermal damage (increasing less than 10°C relative to body temperature). Temperatures inside the interface tissue are sufficiently high to destroy the interface tissue (T>50°C, duration>1 min). Using laser instead of coblation for the removal of interface tissue resulted in higher temperatures - thus a faster removal of interface tissue. This is in accordance with the ablation rate test. Ho:YAG laser is advantageous compared to coblation. We consider Ho:YAG laser a promising tool for interface tissue removal.