Abstract: BACKGROUND: Arthroscopic anchor repair has become the main treatment method for rotator cuff tears at present. Among them, the insertion status of the anchor is a key factor in the success or failure of the operation. However, currently, the impact of the insertion depth of the anchor on the stress of the bone tunnel and the anchor under different bone density conditions remains unclear.
OBJECTIVE: To explore the stress distribution of the bone tunnel and the anchor when the insertion depth of the anchor varies under different bone density conditions by using three-dimensional finite element analysis technology.
METHODS: The CT image data of the humerus of volunteers were collected, and the models of the humerus and the anchor were constructed by using Mimics, 3-Matic, and Solidworks software. In 3-Matic, holes with distances of 0, 2, 4, 6, and 8 mm from the surface of the humerus were respectively created at the same position of the humerus and assembled with the anchor. In Mimics, values were assigned based on the CT gray value to obtain a model with normal bone mass (T value ≥ -1.0). The parameters were changed to construct models with reduced bone mass (-2.5 < T value < -1.0) and osteoporosis (T value ≤ -2.5). In each model, a 70 N pulling force was applied to the anchor along the direction tangent to the inner edge of the bone tunnel. The stress distribution and magnitude of the bone tunnel and the anchor when inserted at different depths under different bone density conditions were observed.
RESULTS AND CONCLUSION: (1) When the insertion depth was the same, as the bone density decreased, the maximum equivalent stress of the anchor increased, while the maximum equivalent stress of the bone tunnel decreased. (2) When the bone density was the same, as the insertion depth of the anchor increased, the maximum equivalent stress of the anchor decreased. When the insertion depth was 4 mm, the stress of the bone tunnel was the smallest and the distribution was relatively uniform. The stress of the anchor was mainly distributed around the lower anchor hole and the proximal thread, and the stress of the bone tunnel was mainly at the part in contact with the proximal thread. The increase in the insertion depth would change the uniformity and pattern of the stress distribution, while the bone density had a relatively small impact on the stress distribution pattern. (3) It is concluded that the bone density of the humerus is crucial for the anchor repair of rotator cuff tears. It is recommended that clinicians measure the bone density of the greater tuberosity of the humerus before the operation. Excessive insertion depth of the anchor does not significantly increase its stability. Clinicians can conduct personalized preoperative assessments by using the finite element analysis method in combination with the actual situation of patients to achieve the best surgical results.

Key words: rotator cuff tear, anchor, insertion depth, bone tunnel, finite element analysis, osteoporosis, orthopedic implant