Evaluating musculoskeletal conditions of the lower limb and understanding the pathophysiology of complex bone kinematics is challenging. Static images do not take into account the dynamic component of relative bone motion and muscle activation. Fluoroscopy and dynamic MRI have important limitations. Dynamic CT (4D-CT) is an emerging alternative that combines high spatial and temporal resolution, with an increased availability in clinical practice. 4D-CT allows simultaneous visualization of bone morphology and joint kinematics. This unique combination makes it an ideal tool to evaluate functional disorders of the musculoskeletal system. In the lower limb, 4D-CT has been used to diagnose femoroacetabular impingement, patellofemoral, ankle and subtalar joint instability, or reduced range of motion. 4D-CT has also been used to demonstrate the effect of surgery, mainly on patellar instability. 4D-CT will need further research and validation before it can be widely used in clinical practice. We believe, however, it is here to stay, and will become a reference in the diagnosis of lower limb conditions and the evaluation of treatment options.

Cite this article: Bone Joint J 2021;103-B(5):822–827.

Software to segment and to analyse connective CT-scan images of the bone-cement-stem complex was developed and validated. Parameters assessed included: volumes (cortical bone, cancelous bone, cement, stem, air in bone and air in cement), cement mantle thickness, cortical & cancelous bone thickness, contact surface area between cement and bone, degree of centralisation (stem in cement, stem and cement in cancelous and in cortical bone). To validate and assess intra- and interob-server reliability two models were implanted in two dried macerated cadaver femurs using a third generation cementing technique. In the first a polished tapered stem (CPT, Zimmer) was cemented and removed after cement curing. The air filled cavity within the cement mantle could be identified as implant, avoiding metallic scatter artefacts. The second model (SLA) used a plastic stem prototype produced by computer design and a rapid prototyping stereolithographic technique. This model does not need to be removed before CT-scanning and allows assessment of whatever femoral implant. Validation occurred by comparing 41 manually segmented femoral cross-sections (25 CPT, 16 SLA) with data of corresponding CT-scan slices. Inter-observer reliability was assessed by having the same person performing the CT-scan and the analysis of both models four times. To assess intra-observer reliability, four different observers segmented 97 representative CT-images (46 CPT, 51 SLA). The average accuracy for surfaces areas (bone, cement, stem) within CT-images was 7.47 mm2 (1.80%), bone & cement mantle thickness: 0.51 mm (9.39%), distances between centroids (stem-cement, stem-bone, cement-bone): 0.38 mm (18.5%) and contours (bone, cement): 0.27 mm (2.57%). The intra- and interobserver reliability of air content in bone and cement was suboptimal (intraclass-correlation coefficient (ICC) as low as 0.54, average ICC: 0.85). All other variables assessed were reliable (ICC > 0.81, average ICC: 0.96). Validity and reliability were comparable when assessed separately for the proximal, middle and distal third of both models. This in vitro technique can assess characteristics of cement mantles produced by different cementing techniques, centralizers and existing femoral implants or stem prototypes.

Using a modern cementing technique, we implanted 22 stereolithographic polymeric replicas of the Charnley-Kerboul stem in 11 pairs of human cadaver femora. On one side, the replicas were cemented line-to-line with the largest broach. On the other, one-size undersized replicas were used (radial difference, 0.89 mm sd 0.13).

CT analysis showed that the line-to-line stems without distal centralisers were at least as well aligned and centered as undersized stems with a centraliser, but were surrounded by less cement and presented more areas of thin (< 2 mm) or deficient (< 1 mm) cement. These areas were located predominantly at the corners and in the middle and distal thirds of the stem. Nevertheless, in line-to-line stems, penetration of cement into cancellous bone resulted in a mean thickness of cement of 3.1 mm (sd 0.6) and only 6.2% of deficient and 26.4% of thin cement. In over 90% of these areas, the cement was directly supported by cortical bone or cortical bone with less than 1 mm of cancellous bone interposed.

When Charnley-Kerboul stems are cemented line-to-line, good clinical results are observed because cement-deficient areas are limited and are frequently supported by cortical bone.