FOOSH injury with screw fixation of the scaphoid

A 20-year-old male patient had undergone surgery with a screw fixation of the scaphoid after a FOOSH (fall on outstretched hand) injury, 5 years ago. A revision surgery of the screw had had to be performed 4 years later, due to non-union of the fracture. A follow-up CT showed unclear bone structure around the screw with the presence of metal artifacts. Further evaluation was needed and performed using an ultra-high resolution (UHR) scan mode (Quantum HD) on a newly installed photon-counting CT (PCCT) scanner (NAEOTOM Alpha®) in our hospital.

The UHR CT images showed a scaphoid fracture with the fixation screw in situ. The fracture in the scaphoid was still largely visible, with a sclerotically bordered fracture line. Minimal central bridging was shown; however, there was no obvious consolidation. Lucency at the tip of the screw, as well as in various parts around the screw, was visualized, signifying screw loosening. The screw protruded proximally into the radiocarpal joint with an impression of slight usuration of the head of the screw into the radius. There were no signs of osteolysis or evidence of migration of the screw. Early osteoarthritic changes of the radiocarpal and the scaphotrapeziotrapezoid (STT) joint were present. A slight dorsal tilting of the lunate was seen with a subtle increase in scapho-lunar distance compared to lunatum-triquetrum distance. In summary, CT findings suggested central, discrete bony bridging, without clear consolidation, and screw loosening. Subsequently, the patient was scheduled for a surgical revision.

Scaphoid fractures due to a FOOSH injury are quite common. Non-union of the fracture may remain if the healing process is compromised which may then impact the wrist function. There are various techniques that can be applied for fracture fixation, one such is a screw fixation. CT follow-ups are often performed to confirm the bony union or nonunion, to assess the screw position and to help surgical planning. [1-3] The opportunities of CT imaging are four-fold: high spatial resolution is required to visualize the trabecular bridging differentiating bony union from non-union; metal artifacts must be minimized to allow assessment of the bone structure around metal implants, confirming or ruling out loosening; low radiation dose is needed to reduce the exposure in repeated scans and last but not least, high quality three-dimensional demonstrations are helpful to determine the intra-scaphoid angle and elements of scaphoid collapse for surgical planning, as well as for an illustrative communication towards clinicians, but also especially to the patients.

This case is performed on NAEOTOM Alpha, a newly developed PCCT scanner, providing energy-resolved CT data at improved spatial resolution, without electronic noise. [4] The spatial resolution is improved by using an ultra-thin slice collimation of 120 x 0.2 mm. Although it is traditionally known that thin-slice images are associated with higher image noise and not suitable for 3D image rendering, it is shown in this case that with PCCT even images acquired at 0.2 mm with low dose and reconstructed with a very sharp kernel of Br89 (with a 2% value of the modulation transfer function of 31 lp/cm) have low image noise. This is facilitated by UHR scan data acquisition at full dose efficiency without additional combs or grids to reduce the detector aperture, and further image noise reduction by a refined model based iterative reconstruction (Quantum Iterative Reconstruction, QIR). Therefore, UHR images are optimal for a photo-realistic 3D visualization using cinematic volume rendering technique (cVRT). Anatomical details such as the trabecular bone structure surrounding the fracture of the scaphoid can be appreciated. Metal artifacts reduction surrounding the screw helps the radiologist to clearly visualize the lucency and confirm the screw loosening with enhanced diagnostic confidence. Furthermore, the electronic noise of a PCCT is eliminated by setting up a predefined digital threshold for counting well above the noise floor. As the electronic noise is dominant at low X-ray flux, its absence has a particular impact on low-dose CT, such as scanning with an integrated special tin filter. This case was acquired at the initial phase of our learning curve on PCCT, when we were still quite conservative about dose reduction. It didn’t take us long to realize the potential of the scanner – we have further reduced the dose by half (3.5–4 mGy), in our routine scan protocol for wrist imaging, and the image quality remains optimal.