Chapter 9, Problem 1TYR

Summary Introduction

Introduction:

A joint is a region where two bones or a bone and a cartilage meet. The humerus is the bone of the upper arm that runs from the shoulder to the elbow. The humerus connects with the scapula to form the shoulder joint. Rotation is the movement of the bone around an axis. Internal rotation occurs when the shoulder revolves towards the midline and external rotation occurs when the movement occurs away from the midline.

Answer to Problem 1TYR

Correct answer:

The internal and external rotation of the humerus occurs because it is a ball-and-socket joint. Therefore, option c is correct.

Explanation of Solution

Justify reasons for the correct statement:

The ball and socket joint is a multiaxial synovial joint that allows movements such as flexion, extension, external and internal rotation in multiple axes.

Option (c) is given as, “ball-and-socket joint”.

The shoulder bone, the humerus is an example for ball and socket joint. The ball and socket joint occurs when the humerus head, which is ball shaped, fits into the socket of another bone, the scapula. It is a highly mobile joint that is involved in both internal and external rotational movements.

Hence, option (c) is correct.

Justify reasons for the incorrect statements:

Option (a) is given as, “pivot”.

Pivot joint is a freely movable synovial joint. It is a uniaxial joint that allows rotational movement only in a single axis. So, it is a wrong answer.

Option (b) is given as, “condylar”.

A condyloid joint is a biaxial joint that allows movement, such as flexion, adduction, and extension in two axes. It does not allow rotational movement. Therefore, it is a wrong answer.

Option (d) is given as, “saddle”.

The saddle joint occurs in the bones that are saddle shaped. It is a synovial joint that allows movements such as flexion, abduction, and circumduction but no rotation. Hence, it is a wrong answer.

Option (e) is given as, “hinge”.

Hinge joint is a type of synovial joint. It allows movements such as flexion and extension in a single axis. So, it is a wrong answer.

Hence, options (a), (b), (d), and (e) are incorrect.

Conclusion

The humerus produces both internal and external rotation because it is a ball-and-socket joint.