Extension of the Shoulder is Essential for Functional Internal Rotation After Reverse Total Shoulder Arthroplasty
These authors point out that the ability to reach behind the body enables critical activities of daily living, such as perineal hygiene, reaching a wallet in the back pocket, tucking in a shirt and fastening a bra. They observe that disabling loss of the ability to reach behind the body occurs frequently after reverse total shoulder arthroplasty (RTSA).
They proposed that limitation of reach behind the body after RTSA may not primarily related to a deficit of glenohumeral internal rotation but rather due to a lack of humerothoracic extension.
Fifty patients having RTSA were examined with special attention to reach behind the body: “reaching the back pocket”, “personal toileting hygiene”, “tucking a shirt into the pants behind the back”, “pulling-up pants behind the back”. For analysis, patients were divided into a group with poor reach behind the body (n=19) and a group with good reach behind the body (n=31).
Active extension of the arm in relation to the body for the opposite shoulders was similar in both groups (60±11 and 66±14) (measured as shown below).
After RTSA, active humerothoracic extension on the operated side averaged 55±14 degrees in the shoulders with good reach behind the body and 39±11 degrees
in the shoulders with poor reach behind the body. The differences in extension
between the RTSA side
and the contralateral side averaged 11 degrees for those with
good reach behind the body and 21 degrees in the shoulders with
poor reach behind the body.
No patient with good reach behind the body had active humerothoracic extension less than 40 degrees.
The ability to reach behind the body correlated better with humerothoracic extension than with "internal rotation" measurements.
The authors concluded that good reach behind the body after RTSA requires at least 40° of humerothoracic extension. If reach behind the body is unsatisfactory despite 40° of humerothoracic extension, restriction of internal rotation in full extension may be the limiting factor.
Comment: This is an important paper because it points out that while the ability to reach various anatomic landmarks behind the body is often referred to as "internal rotation",
the situation is a bit more complex because functions behind the body depend on humerothoracic and humeroscapular motion
The importance of distinguishing humerothoracic and humeroscapular motion was pointed out in two articles published in 1992, Laxity of the normal glenohumeral joint: A quantitative in vivo assessment and A system for describing positions of the humerus relative to the thorax and its use in the presentation of several functionally important arm positions. In these papers, the motion of the humerus relative to the thorax and the motion of the humerus in relation to the scapula were tracked separately and simultaneously in living subjects using electromagnetic receivers attached to pins inserted in the scapula (S), and humerus (H), while a transmitter was fixed to the thorax (T). Direct attachment of the sensors to bone avoided the artifacts associated with markers placed on the skin. This system enabled real-time recording of the six-degree-of freedom relationships among the humerus, scapula and thorax during active motions in vivo.
Some of the key findings from these studies are presented in the 1994 book,
Practical Evaluation and Management of the Shoulder - a free copy of which can be obtained
here.
The plane of humeral motion in relation to the plane of the body along with the degrees of elevation was recorded for each of the common activities of daily living.
The results can be displayed on a global diagram in the lateral and frontal projections as shown below.
Here are the humerothoracic positions for common activities. Note that in contrast to the other activities, reach behind the body (tucking in the shirt behind - gold dot in the figure below) required elevation in extension (i.e. in a plane posterior to the plane of the body). All of the other functions were performed in planes anterior to the plane of the body.
This result is also shown in the table below
The humerothoracic planes of elevation needed for seven of the Simple Shoulder Test functions are shown below.
The discussion above pertains to motion of the humerus relative to the plane of the body.
A similar analysis can be performed for the motion of the humerus relative to the plane of the scapula.
and displayed on humeral scapular global diagrams in which the plane of the scapula is the reference.
Note that reach up the back (tucking in the shirt) requires humeroscapular extension (gold dot).
Thus, reach behind the body requires humerothoracic extension (elevation in a plane posterior to the plane of the body) which is provided by humeroscapular extension (elevation posterior to the plane of the scapula). This is because the ability of the scapula to "extend" on the thorax is limited.
This analysis of in vivo relative motions of the humerus, scapula and thorax during functions of daily living also reveals that reach up the back (tucking in a shirt) also requires internal rotation of the humerus relative to the scapula (gold dot).
From the foregoing we can conclude that the functionally import motions requiring reach behind the body depend on both humerothoracic extension (which relates closely to humeroscapular extension) and humeroscapular internal rotation.
In trying to optimize the function of our patients having RTSA, we should consider whether tightening the strap muscles (which lie anterior to the glenohumeral joint) during RTSA may be one of the potential causes of limited extension and whether contact between the lesser tuberosity and the glenoid may be one the potential causes of limited humeroscapular internal rotation.
As always, there's a trade off. The combination of adduction, internal rotation, and extension is recognized as the position in which dislocation of a RTSA is most likely: "The simple act of reaching behind to scratch the middle of your back or (for women) undoing a bra-strap can dislocate a reverse shoulder implant. This action places the arm in a position of extension, adduction (arm close to the body), and internal rotation." (see this link and this link and this link).
