Stemless humeral head components - the risk of overstuffing.

The concept of overstuffing was introduced in 1994 in the book, "Practical Evaluation and Management of the Shoulder" (see pages 181-187 in this link). This text pointed out that to avoid persistent stiffness after shoulder arthroplasty, the surgeon must perform adequate capsular releases and avoid component malposition or  components that put too much prosthesis volume into the joint so that it is tightened or "overstuffed". 

The authors of Radiographic assessment of prosthetic humeral head size after anatomic shoulder arthroplasty measured the variation between surgeons and between types of prosthetics in reproducing the anatomic center of rotation (COR) of the humeral head after anatomic shoulder arthroplasty. They studied the anteroposterior radiographs of 125 stemmed and 43 resurfacing shoulder arthroplasties, performed by 5 experienced surgeons, were analyzed. All patients had primary replacement for treatment of end-stage glenohumeral arthritis. 

A best-fit circle (yellow circle in the figure below) was placed on the AP image with three preserved bone landmarks

: the lateral cortex of the greater tuberosity, the medial calcar at the inflection point where calcar meets the articular surface, and the medial edge of the greater tuberosity at the medial supraspinatus insertion. A second circle (blue circle in the image below), the implant-matched circle, was placed to fit the curvature of the prosthetic humeral head. The COR was then identified from each circle, and the distance, in millimeters, between the CORs of the anatomic and implant circles was calculated.

A difference in COR of >3.0 mm was considered clinically significant and analyzed for the cause of this deviation.

The average deviation of the postoperative COR from the anatomic COR was 2.5 ± 1.6 mm for stemmed cases and 3.8 ± 2.1 mm for resurfacings. 

Thirty-nine stemmed cases (31.2%) and 28 resurfacings (65.1%) were beyond 3.0 mm of deviation and regarded as outliers. 

The majority of the stemmed outliers and all resurfacing outliers were overstuffed. An improper humeral head size selection and inadequate reaming were the main reasons for the deviation in stemmed and resurfacing outliers, respectively.

Resurfacing arthroplasty exhibited significantly greater deviations compared with stemmed arthroplasty (P < .001), indicating that surgeons have more difficulty in restoring the anatomy with resurfacings. 

The difficulty in placement of a stemless humeral component is again pointed out in another article:

Does Computerized CT-based 3D Planning Of The Humeral Head Cut Help To Restore The Anatomy Of The Proximal Humerus After Stemless Total Shoulder Arthroplasty?

They measured deviation from the premorbid center of rotation (COR); more than 3mm was considered as minimal clinical important difference. Additionally, pre- and postoperative humeral head diameter (HHD), head neck angle (HNA) and humeral head height (HHH) were measured.

The mean distance from of the premorbid to the implanted head COR was 4.3mm ± 3.1mm. Thirty five shoulders (35%) showed a deviation of less than 3mm (mean 1.9, ±1.1) and 65 shoulders (65%) a deviation of ≥3mm (mean 8.0 ± 3.7). The medial deviation of the humeral head COR of ≥3mm in relation to premorbid COR was considered as over overstuffing. A poorly performed humeral head cut was the main reason for overstuffing which was seen in 88% of the cases with inaccurate poor anatomic reconstruction. Preoperative small HHD, low HHH and varus angulated HNA are risk factors for poor anatomic reconstruction after stemless TSA.The level of the humeral head cut was responsible for overstuffing in the 46  of the 57 overstuffed cases. The preoperative HHD, HHH and the HNA were significantly larger, higher and more in valgus angulation in the group with accurate compared to the 24 group with poor restoration of premorbid anatomy.The authors concluded that restoration of proximal humeral anatomy after stemless TSA using CT-based 3D planning was not precise. 

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