Quantitative imaging of bone remodeling in patients with a unicompartmental joint unloading knee implant (ATLAS Knee System)-effect of metal artifacts on a SPECT-CT-based quantification.

BACKGROUND: SPECT-CT using radiolabeled phosphonates is considered a standard for assessing bone metabolism (e.g., in patients with osteoarthritis of knee joints). However, SPECT can be influenced by metal artifacts in CT caused by endoprostheses affecting attenuation correction. The current study examined the effects of metal artifacts in CT of a specific endoprosthesis design on quantitative hybrid SPECT-CT imaging. The implant was positioned inside a phantom homogenously filled with activity (955 MBq 99mTc). CT imaging was performed for different X-ray tube currents (I = 10, 40, 125 mA) and table pitches (p = 0.562 and 1.375). X-ray tube voltage (U = 120 kVp) and primary collimation (16 × 0.625 mm) were kept constant for all scans. The CT reconstruction was performed with five different reconstruction kernels (slice thickness, 1.25 mm and 3.75 mm, each 512 × 512 matrix). Effects from metal artifacts were analyzed for different CT scans and reconstruction protocols. ROI analysis of CT and SPECT data was performed for two slice positions/volumes representing the typical locations for target structures relative to the prosthesis (e.g., femur and tibia). A reference region (homogenous activity concentration without influence from metal artifacts) was analyzed for comparison. RESULTS: Significant effects caused by CT metal artifacts on attenuation-corrected SPECT were observed for the different slice positions, reconstructed slice thicknesses of CT data, and pitch and CT-reconstruction kernels used (all, p < 0.0001). Based on the optimization, a set of three protocols was identified minimizing the effect of CT metal artifacts on SPECT data. Regarding the reference region, the activity concentration in the anatomically correlated volume was underestimated by 8.9-10.1%. A slight inhomogeneity of the reconstructed activity concentration was detected inside the regions with a median up to 0.81% (p < 0.0001). Using an X-ray tube current of 40 mA showed the best result, balancing quantification and CT exposure. CONCLUSION: The results of this study demonstrate the need for the evaluation of SPECT-CT protocols in prosthesis imaging. Phantom experiments demonstrated the possibility for quantitative SPECT-CT of bone turnover in a specific prosthesis design. Meanwhile, a systematic bias caused by metal implants on quantitative SPECT data has to be considered.

ACL graft migration under cyclic loading.

Elongation and migration of ACL grafts will lead to a deterioration of the initial stability of ACL reconstructions. The graft migration has been sparsely investigated independently from the elongation of the graft-fixation complex. The hypothesis of this investigation was that cyclic tensile loads cause a measurable migration of the grafts. Three graft/fixation combinations were investigated in human femora (n = 7): human bone-patellar tendon grafts fixed with a biointerference screw (BPTG-IS) and free tendon grafts (porcine) fixed with either a Bio-TransFix pin (FTG-TF) or an Endobutton CL (FTG-EB). The grafts were fitted with tantalum markers. Then, the specimens were repetitively loaded (50-250 N, 800 cycles). The marker position was fluoroscopically determined at defined intervals and the migration calculated from the change in position relative to a fiducial marker within the bone. A migration of the grafts occurred in all three groups. The migration in the FTG-EB group was significantly larger than in the two other groups (P < 0.01). After 800 cycles, average migration was 0.3 (+/-0.2) mm in the BPTG-IS group, 0.7 (+/-0.4) mm FTG-TF group, 2.0 (+/-1.3) mm in the FTG-EB group. This migration might contribute to a loss of initial stability. Because the graft migration was dependent on the technique, the presented data might provide additional arguments for making the decision on the most appropriate graft/fixation combination.

Tensile forces on sutures in the human lateral knee meniscus.

Tensile strength is the most often reported parameter in biomechanical investigations of meniscal repair techniques. However, the magnitude of the tensile forces that actually occur on repaired lesions is not clear. The purpose of this study was to investigate if tensile forces occur on repaired lateral meniscal lesions, which could exceed the failure strength of common repair techniques. In human knees (n = 6), vertical-longitudinal lesions 25 mm in length were created in the posterior horn of the lateral meniscus at a distance of 3 mm from the meniscosynovial junction and the popliteal hiatus. A braided steel wire, resembling a vertical suture, was inserted into the meniscal tissue and fitted with a force transducer. The knees were mounted in an apparatus, which simulated weight bearing and non-weight bearing conditions. Repeated measurements were conducted with both internal and external rotation at flexion angles of 0 degrees , 30 degrees , 60 degrees , 90 degrees and 120 degrees . Weight loading alone caused no tension on the suture. Combined flexion and rotation generated mean forces between 0.5 and 4.1 N. No significant effect of the flexion angle or direction of rotation was found. If a minimum strength of 10 N was assumed for the common meniscal repair techniques, the tensile forces were well below this limit under all circumstances (P < 0.001). These data indicate that, within the range of motion investigated, no significant tensile forces occur on longitudinal lateral lesions. Forces other than tension and biological factors are of greater importance for the healing. Therefore, the assessment of repair techniques should not be based on alone the ability to resist high distraction forces.

The extent of laceration of circumferential fibers with suture repair of the knee meniscus. Winner of the AGA DonJoy award 2006.

INTRODUCTION: Cannulas used with suture based meniscal repair techniques can potentially injure the load transmitting fibers of the meniscus. The subject of this study was to quantify this effect in a porcine in vitro model. MATERIALS AND METHODS: From fresh frozen medial porcine menisci tissue specimens were harvested following the course of the peripheral circumferential fibers bundles. In the first part of the study the tissue samples were perforated with the cannulas of either a Fast Fix or a Rapidloc device or with an 18-gauge needle. The specimens were then visually inspected for fiber damage using low power microscopy and the mean size of the laceration was measured. Finally, the extent of the tissue laceration was indirectly determined using non-contact strain measurements of the samples before and after puncture. RESULTS: When advanced with the cutting edge perpendicular to the fibers, the cannulas consistently cut the fibers while those were rather separated with the opposite orientation. It could be shown that specimens with a mean width of 8.1 mm lost 25% of the load transmitting cross section when being perforated two times with a Fast Fix device (P < 0.001). This effect is negated when the cannula was oriented in line with the fibers. CONCLUSIONS: Cannulas used for suture based meniscal repair can cause a substantial laceration of the meniscal tissue. The effect strongly depends on the orientation of the cutting edge of the cannula relative to the course of the fibers and can thus potentially be avoided by an appropriate handling and design.

The effect of meniscus compression on the biomechanical properties of repaired meniscal lesions. Winner of the AGA-DonJoy Award 2003.

INTRODUCTION: The objective of the present study was to investigate the influence of meniscus compression, as it occurs with weight-bearing, on the biomechanical behavior of different repair types. MATERIALS AND METHODS: Young human meniscal tissue specimens were fitted with a single Bionx Arrow, a Innovasive Meniscal Screw, or a vertical suture (Ethibond 2-0) according to standard clinical practice. Test specimens then underwent repetitive loading with 10 N under 1 N pretension up to a maximum of 1182 cycles and were finally loaded to failure. Half of the cases in each group were additionally subjected to synchronized intermittent compression during the test. The main variables investigated were the number of cycles survived and ultimate failure load. To reduce the complexity, investigations were limited to the peripheral repair site. RESULTS: The number of cycles survived was significantly affected by compression and the type of repair. Arrows and Screws survived 311 and 52 cycles, respectively, without compression, while 983 and 1182 cycles were survived with added compression. In the suture group, no premature failure occurred either with or without compression. Both compression and the type of repair had a significant effect on the ultimate failure load. It increased about twofold in the Arrow and Screw group, while the suture group was not affected. CONCLUSIONS: Compression of the meniscus can substantially increase the pullout resistance of meniscal repair implants and thus seems not to be a factor negatively influencing the stability of the repair.