Osteochondritis dissecans of the capitellum

A 13-year-old female patient, a gymnast, with left elbow complaints for the past 18 months, presented herself to our hospital. A radiograph of the elbow showed a suspicious osteochondritis dissecans (OCD) of the capitellum. To further evaluate the dimension of the lesion and the possible existence of loose fragments, an ultra-high resolution (UHR) scan mode (Quantum HD) on a photon-counting CT (PCCT) scanner (NAEOTOM Alpha®) was requested.

The UHR CT images showed a cortical defect at the capitulum of the left humerus, with an approximate size of 9.2 mm (coronal) x 9.8 mm (sagittal) x 4.1 mm (depth) on the medial side. There were two loose fragments – one was about 5.1 mm in size, next to the capitulum, and the other was about 6.2 mm in size, seen dorsally on the lateral side of the olecranon. There were no signs of any focal osseous abnormalities elsewhere, nor evidence of focal muscle abnormalities. The trabeculation of the radial head was normal. CT findings suggested an OCD focus at the capitellum of the left humerus with two loose bone fragments, demonstrating an unstable lesion. Subsequently, the patient was scheduled for arthroscopic surgery.

OCD is a disorder of the articular cartilage and subchondral bone which most commonly occurs in adolescent athletes. [1] In the elbow, it typically affects the humeral capitellum. Early recognition and appropriate intervention may protect adolescents from fragmentation of the OCD lesion and the development of irreversible cartilage damage. It is important to differentiate between stable and unstable OCD lesions, hereby is a CT considered more sensitive than a radiograph, to determine the best treatment option. [2] An unstable lesion would require a surgical approach, and prior to that, it would be essential to acquire detailed diagnostic information, such as the lesion size, the stability and viability of the lesion fragment and the number of present loose bodies, for surgical planning. 

This case is performed on a NAEOTOM Alpha, a newly developed dual source PCCT scanner, providing energy-resolved CT data at improved spatial resolution, without electronic noise. [3] UHR images are acquired at 0.2 mm slice width and reconstructed using a very sharp kernel (Br89) with a 2% value of the modulation transfer function of 31 lp/cm. This provides improved visualization of the cortical and trabecular bone microarchitecture, enhancing the diagnostic confidence of the radiologists. Traditionally, with conventional CT, UHR images are associated with higher image noise and require higher radiation dose. However, PCCT enables UHR scanning without substantial increase in image noise, because UHR scan data are acquired at full dose efficiency without additional combs or grids to reduce the detector aperture, and image noise is further reduced by a refined model based iterative reconstruction approach (Quantum Iterative Reconstruction, QIR). The low image noise even enables the use of the UHR images for cinematic volume rendering technique (cVRT), providing an excellent photo-realistic 3D visualization and facilitating surgery planning. Potentially, image noise reduction also leads to a reduction of the radiation dose, which is important especially for young patients. As shown in this case, the combination of improved spatial resolution with reduced image noise, provided by UHR imaging with PCCT, greatly benefits the musculoskeletal imaging.