Morphometric study of the thoracic vertebral pedicle in an Indian population.

Study Design: Morphometric study of the thoracic vertebral pedicular parameters and comparison with the previous studies in the literature.

Objectives: To define pedicular dimensions and screw placement in the Indian population.

Summary Of The Background Data: Pedicular morphometric characteristics vary in different population groups. Thoracic pedicular screw fixation is being used more frequently with the advent of better imaging methods. Because of the small size and close proximity to the neurovascular structures, thoracic pedicle fixation has little margin of safety.

Methods: T1-T12 vertebral pedicles were studied in 18 cadavers by direct, roentgenographic, and computerized tomographic scan methods. The following parameters were studied: transverse diameter, transverse angle, sagittal diameter, sagittal angle, chord length, interpedicular distance, pedicle entrance point, and pedicle cortical thickness (medial and lateral). RESULTS.: Transverse diameter was more than 6 mm at both ends of the thoracic spine (T1, T2, T11, and T12). Between T3-T9 levels, it was less than 5 mm at some levels. The transverse angle was widest at T1 (30 degrees) and was less than 5 degrees from T5 to T12. Pedicles were directed cephalad in the sagittal plane at all thoracic levels. Sagittal angle was less than 12 degrees at all thoracic levels. Chord length was largest at T11 (37.3 mm) and smallest at T1 and T2 (29.9 mm). Interpedicular distance was 29 mm at T12 and 21.3 mm at T1. Medial pedicular cortex was thicker than lateral cortex at all levels.

Conclusions: These results suggest that even a 4-mm screw should be used carefully at the midthoracic level; 5-mm screw seems to be safe at upper and lower thoracic spine. Because of very small sagittal and transverse angle at mid and lower thoracic levels, the pedicular screw should be inserted along almost perpendicular line in these planes; 25-mm and 30-mm screw length appears to be safe at upper thoracic and lower thoracic levels, respectively. Pedicle entrance point lies along the midtrans-verse line at upper thoracic levels and along the upper border of transverse process at lower thoracic levels.