Increased Moment of Inertia in the Bell Clapper Anomaly as the Underlying Mechanism of Testicular Torsion demonstrated using a 3D Printed Model
Austen Gregory Te, BS1, Michael Sun, MD2, Young Son, MD3, Gregory Dean, MD4.
1Lewis Katz School of Medicine, Philadelphia, PA, USA, 2Drexel University, Philadelphia, PA, USA, 3Jefferson University, Urology Voorhees USA, Philadelphia, PA, USA, 4Temple University, Urology Voorhees USA, Philadelphia, PA, USA.
BACKGROUND: Testicular torsion is a serious genitourinary emergency placing adolescent boys at risk. Without rapid diagnosis and surgical correction, boys can undergo testicular loss in less than six hours. The Bell Clapper deformity is a congenital anomaly correlated with increased rates of testicular torsion. We have developed a mechanical model explaining the role of the transverse orientation of the bell clapper anomaly plays in testis torsion. This is based on the increased moment of inertia when the testis is in a transverse position. We have separately documented that the onset of pain in a group in 190 patients with testis torsion rising at 1 am and peaking at 9 am supporting the association with sleep and the supine position. Individuals roll in bed an average of 20 times per night (Iber et al 2007). We hypothesize that a bell clapper deformity leads to an increased moment of inertia and as individuals roll in their sleep, this results in axial misalignment between the cord structures of the testis and the body. We develop a 3D printed model to confirm the increased moment of inertia when the testis is oriented transversely as with a bell clapper anomaly.
METHODS: We performed dimensional analysis using ultrasound patient data in 14 year old boys undergoing routine scrotal ultrasounds at St. Christopher’s Hospital for Children. The mean dimensions were then used to construct a 3D printed model which was used for mechanical analysis. Models were oriented in the long and transverse axis after axial pins were placed within the models to facilitate rolling analysis on an incline. The models were then rolled down a 3D printed 30cm ramp at 10.5 degrees and recorded within a 17cm window. The duration of time it took from release to the end of the ramp was broken down frame by frame to be accurate to the hundredth of a second using iCloud’s Photos application. Rolling duration is a measure of kinetic rotational energy. Increased duration required to transit the 17cm window is associated with an increased moment of inertia.
Results: The transverse orientation (bell clapper) resulted in a mean rotational time of 1.71s vs a longitudinal position (normal) of 1.27s. There was a strong correlation (r = 0.969) between rolling duration and moment of inertia of all models while there was no significant correlation between rolling duration and volume (r = 0.07).
CONCLUSIONS:
We propose a model of testicular torsion whereby a transverse or bell-clapper orientation results in increased moment of rotation. As the body rolls during normal sleep, we suggest that the testis undergoes axial misalignment resulting in testicular torsion.
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