Can surgeons optimize range of motion and reduce scapulohumeral impingements in reverse shoulder arthroplasty? A computational study

Abstract

Background

Early glenohumeral impingement leads to poor range of motion and notching in reverse shoulder arthroplasty. The aim was to find from planning software which implant configuration provides the best motions in reverse shoulder arthroplasty.

Patients and Methods

Reverse shoulder arthroplasty planning (Glenosys) was made in 31 patients (12 men, 19 women, 76 ± 6 yo) and impingements were analyzed. Inlay (155°-inclined) and Onlay (145°-inclined) humeral designs were tested. Four configurations were tested for each shoulder: “INLAY”: non-lateralized glenoid-inlay humerus, “BIO-INLAY”: lateralized glenoid (BIO-RSA)-inlay humerus, “ONLAY”: non-lateralized glenoid-onlay humerus, and “BIO-ONLAY”: lateralized (BIO-RSA) glenoid-onlay humerus.

Results

BIO-ONLAY and BIO-INLAY groups presented a significantly better result in all tested motion (p < 0.001 for all tests). BIO-ONLAY allowed a significantly better external rotation, extension and adduction than BIO-INLAY with decreased impingements with the pilar. BIO-INLAY presented a significantly better abduction. In abduction, an abutment of the greater tuberosity against the acromion was associated with a lower range of motion (p < 0.0001) and did not depend on the lateralization.

Conclusion

Glenoid lateralization delays the glenohumeral impingement in reverse shoulder arthroplasty and gives the best rotations, adduction and extension when associated with neutral inclination and humeral 145° inclination. Greater tuberosity abutment has to be avoided in abduction and the Inlay design provides the best abduction.

Introduction

The original design of the Grammont reverse shoulder arthroplasty (RSA) was based on two innovative concepts: (1) a 155° inclined humeral implant with a polyethylene (PE) cup located inside the bone (Inlay design) in order to tension the deltoid muscle fibers, and (2) a medialized glenoid component with the center of rotation located at the glenoid surface to decrease the shearing forces and the risk of glenoid implant loosening. ¹ This implant configuration has led to the success of the reverse prosthesis but has its known limits: the medialization together with the horizontalization (inclination at 155°) of the humeral component have been the cause of scapular impingement and complications: high rate of inferior scapular notching, decreased range of motion (ROM) in rotation and increased risk of prosthetic instability.^2–5 In 2006, with the goal of reducing the rate of scapular notching and improving the passive range of the humeral cup around the glenoid sphere, we proposed a new surgical technique to lateralize the glenoid implant. The “BIO-RSA technique” (bony increased reverse shoulder arthroplasty) which consists of bone grafting the glenoid by placing a disk of cancellous bone graft, harvested in the proximal humerus, under the baseplate and obtain healing to the native glenoid.^6,7 Our early results, published in 2011, have confirmed the benefits provided by this concept with a lower rate (20%) of scapular notching, an improved range of shoulder motion, and an excellent rate (97%) of bone graft incorporation. ⁷ In 2010, with the goal of lateralizing the glenoid and correcting the superior inclination of the glenoid, we started using an angulated bone graft (instead of a symmetrical bone graft). The “Angled-BIORSA” technique was designed to obtain neutral alignment of the glenoid implant in the vertical plane (i.e., an RSA angle equal to 0°). ³ In the meantime, some new convertible humeral implants have evolved from the original Inlay-155° design toward convertible humeral implants with an Onlay design (i.e., with the PE cup over the plane of the humeral bone cut) and decreased humeral implant inclination (145°). ⁸ We first used the “Angled BIORSA technique” with the original Grammont humeral design (Inlay-155°, Aequalis Reverse™, Tornier-Wright) and more recently with the new Onlay-145° humeral implant (Aequalis Ascend™, Tornier-Wright). The techniques of prosthesis positioning, as well as the choice of the implant design for RSA have evolved in our department through the years in an attempt to maximize postoperative mobility and reduce scapulohumeral impingement. However, the difference between humeral Inlay and Onlay designs, the role of glenoid lateralization and the role of inclination of the glenoid and humeral implants have been poorly documented in the literature. For the last few years, we have been using a software (Blueprint™, Imascap-Wright) to preoperatively plan our shoulder arthroplasties.^9,10 This software allows planning the implant’s optimal size and positioning, and as well anticipating the optimal postoperative ROM in RSA. By using the automated software, we sought to objectivize the evolution our daily practice from the 2000s and highlight the benefits given to our patients requiring RSA with respect to mobility through the change in implant design and the evolution in the implantation technique. Therefore, the purpose of the present study was to compare the ROM in RSA with between different humeral implant designs (Inlay-155° and Onlay-145°) and a glenoid implant that could be lateralized or not (RSA vs. BIO-RSA) using Blueprint™ software that represents the evolution of our practice.

The question we sought to answer was: what implant configuration allows us to obtain the best ROM in RSA?

Methods

It was a computational study (IRB: Ref. Study 2017-06) performed using a validated software (Blueprint™—Wright-Imascap), on 31 shoulders with cuff tear arthropathy (CTA) scheduled for a RSA. In order to limit the influence of glenoid morphology, we selected only concave shaped glenoid (E1-type according to Favard et al.), with retroversion between 0° and 15° and inclination between 0° and 20°. There were 19 females and 12 men with average age of 76 ± 6 years old. The mean glenoid retroversion was 8 ± 4° and mean global inclination 10 ± 4°. The RSA-angle which represents the inclination of the glenoid was 19 ± 4°. ¹¹

Planning

On the glenoid side, a centered glenosphere was always chosen and the baseplate was placed flush against the inferior glenoid rim. The baseplate and glenosphere were matched to obtain minimum 5 mm of inferior overhang: a 25-mm baseplate was combined with a 36-mm sphere for small glenoids (all women), whereas a 29-mm baseplate was combined with a 42-mm sphere for large glenoids (all men). The objective was to position the central peg centered in the glenoid vault. On the various planning performed on the same shoulder, the same entry point and the same version, that allowed to center the peg in the glenoid vault, were chosen for the central peg of the baseplate. The accepted limits of retroversion range from 0° to 10° and was constant for the same shoulder. A minimum of 80% of baseplate seating on the native glenoid was required.

Two techniques were chosen to implant the glenoid (Figure 1(a)).

1. Without lateralization: the baseplate was positioned on the lower side of the glenoid. Its inclination was determined using the available ancillary that provided an inferior 10°-tilted drill guide, which equated to “RSA-angle”—10°. The “RSA-angle”, described by Boileau et al., is defined as the angle between the inferior glenoid fossa (where a RSA baseplate would be implanted) and the orthogonal line to the floor of the supraspinatus fossa. ¹¹ Thus, this implant position was obtained by strictly following the ancillary provided by the manufacturer.

2. With lateralization: the “angled BIO-RSA” technique, with a 12.5° angulated graft with maximal highest depth at 10 mm, was always used. ³ All the inclination of the graft was in the vertical plane. The baseplate was positioned on the lower side of the glenoid and with an inclination at 0°, that is perpendicular to the floor of the supraspinatus fossa (RSA-angle = 0°).

Figure 1.

Planning of (a) the glenoid with or without lateralization and (b) the humerus.

The diameter of the glenosphere was chosen according to the gender: 36 mm for women, and 42 mm for men.

On the humeral side, two different implant models were tested (Figure 1(b))

1. The Inlay-155° model (Aequalis Reverse™ model, Wright) was positioned so that the cut was performed on the medial side of the greater tuberosity (GT) and that the metaphysis stood on the calcar. No modularity on the metaphysis was available.

2. The Onlay-145° model (Ascend Flex model, Wright) was positioned so that the cut was performed on the medial side of the GT and just over the calcar. The tray was always eccentric (1.5 for women, 3.0 for men). When lateralization was achieved on the glenoid side, the index was on the 6th position so the humerus was lengthened (medial eccentric tray). When no lateralization was achieved on the glenoid side, the index was on the 12th position so the humerus was lateralized (lateral eccentric tray).

The size of the humeral metaphysis (36 or 42) was adapted to the chosen glenoid implant with the same standard PE for all the planned implants. All the plannings were done by a single surgeon (MOG).

Groups and range of motion

We planned Inlay and Onlay humeral designs with or without an angled BIO-RSA for each shoulder. Thus, four groups (4 × 31 shoulders = 124 plannings) were constituted: (Figure 2)

• INLAY Group: Inlay humeral implant with a non-lateralized glenoid implant (Figure 2(a))

• ONLAY Group: Onlay humeral implant with non-lateralized glenoid implant (Figure 2(b))

• BIO-INLAY Group: Inlay humeral implant with a lateralized glenoid implant (Figure 2(c))

• BIO-ONLAY Group: Onlay humeral implant with a lateralized glenoid implant (Figure 2(d))

Figure 2.

Four iterations of planning were performed for the 31 cases: (a) INLAY-group, (b) ONLAY-group, (c) BIO-INLAY group, and (d) BIO-ONLAY group.

At the end of the planning, the ROM of each arthroplasty was obtained. We noted the flexion, the extension, the adduction, the abduction, the external, and internal rotations. This motion was only considered in the glenohumeral joint as the scapulothoracic joint was not modeled. A gender analysis was also performed.

Bony impingement

The automatic determination of the ROM made it possible to analyze various impingements that can occur during arm motion. For each motion, the impingement zone was highlighted by the software (red spot) and noted. We analyzed the association between the impingement zone and the motion to determine the best implant design and its optimal positioning.

Statistics

All statistical analyses were performed using MedCalc 12.0 software (MedCalc Software bvba, Ostend, Belgium). Pairwise comparisons between the groups were performed to correlate each prosthetic design to the final ROM. The same analysis of variance was performed to examine the relationship between each of the impingements and ROM. The χ ² test was used to compare the localization of the abutment according to the group. An unadjusted α of 0.05 level of significance was used for all tests.

Results

Range of motion

The results for ROM are shown in Table 1.

Table 1.

Range of motion according to the implant designs.

ROM (°)	INLAY	ONLAY	BIO INLAY	BIO ONLAY
Flexion	86 ± 12	101 ± 22	128 ± 25	125 ± 24
Extension	31 ± 51	21 ± 32	38 ± 33	100 ± 28
External rotation	2 ± 25	22 ± 14	42 ± 15	62 ± 12
Internal rotation	18 ± 29	74 ± 22	73 ± 15	84 ± 15
Abduction	88 ± 7	73 ± 11	100 ± 12	87 ± 16
Adduction	1 ± 2	8 ± 6	16 ± 8	32 ± 6

For each design of the humeral implant, the ranges of motion were significantly improved after the lateralization of the glenoid by a BIO-RSA (p < 0.001 for INLAY-BIOINLAY and ONLAY-BIO ONLAY comparisons) in adduction-abduction, in flexion-extension, and in internal-external rotations (Figure 3).

Figure 3.

Assessments of the ROM according to the various plannings. ROM: range of motion; ER: external rotation; IR: interne rotation.

Glenoid lateralization was found to be an independent factor for improved abduction in both humerus designs. Moreover, it was noticed that the 100°-adduction/abduction amplitude threshold could only be achieved with the lateralized glenoid designs. Irrespective of glenoid lateralization, Inlay designs gave better abduction than Onlay. However, this was not the case with adduction where the 155°-inclination of the Inlay design induced an earlier abutment with the pilar (Table 2).

Table 2.

ROM comparisons related to the humerus design and the lateralization.

ROM (°)	INLAY + BIO INLAY	ONLAY + BIO ONLAY	P	INLAY + ONLAY	BIO INLAY + BIO INLAY	p
Flexion	107 ± 28	114 ± 24	0.008	94 ± 19	128 ± 22	<0.001
Extension	35 ± 43	61 ± 49	0.002	26 ± 43	69 ± 43	<0.001
External rotation	22 ± 23	42 ± 24	<0.001	12 ± 15	52 ± 17	<0.001
Internal rotation	45 ± 36	79 ± 19	<0.001	46 ± 38	78 ± 16	<0.001
Abduction	94 ± 12	80 ± 16	<0.001	80 ± 12	93 ± 16	<0.001
Adduction	9 ± 10	20 ± 13	<0.001	4 ± 6	24 ± 11	<0.001

In flexion, a significant improvement was noted with glenoid lateralization (94 ± 19° vs. 128 ± 22°, p < 0.0001) but the humeral design did not have any significant influence. An optimal and significantly better extension range was only achieved by the combination of a lateralized and an Onlay design (100 ± 28°, p < 0.0001).

Lateralization and Onlay designs were both independent factors that improved rotation. With a 155° inclination and no lateralization, the Inlay implant achieved the worst motion in rotation (20 ± 35°). For the same glenoid implant and whatever was the lateralization, the Onlay design provided better mobility in rotation than the Inlay design and the reduced implant inclination was the only factor that can explain this improvement. Compared to the Inlay design, the use of the Onlay humerus with its inclination at 145° allowed a significantly major improvement in rotation (95 ± 36°, p < 0.0001) and additional lateralization on the glenoid side achieved the best rotations observed (146 ± 27°, p < 0.0001). Glenoid lateralization with an Inlay design showed better rotations than the non-lateralized Onlay design (115 ± 31° vs. 95 ± 36°, p = 0.0008). Gender ROM analysis showed that glenoid lateralization was more efficient in women when using an Inlay design in adduction, extension, and external rotation but also in internal rotation. No differences were observed in the other groups (Figure 4).

Figure 4.

Gender analysis of the range of motion in the four groups. ADD: adduction; ABD: abduction; IR: internal rotation; ER: external rotation; Ext: extension; Flex: flexion. *Statistically different.

Impingement

Impingement varied according to the observed ROM (Table 3).

Table 3.

Repartition of the impingement zones between the humerus implant and the scapula according to the implant design.

	INLAY			ONLAY			BIOINLAY			BIOONLAY
Adduction
	Pilar/Neck	31	(100%)	Pilar/Neck	31	(100%)	Pilar/Neck	31	(100%)	Pilar/Neck	31	(100%)
Abduction
	Ant/Sup glenoid	23	(74%)	GT	27	(87%)	Lat Hum	23	(74%)	GT	28	(90%)
	Lat Hum	8	(26%)	Ant/Sup glenoid	3	(10%)	GT	5	(16%)	Lat Hum	3	(10%)
				Lat Hum	1	(3%)	Ant/Sup glenoid	3	(10%)
Internal rotation
	Pilar	31	(100%)	Pilar/Neck	12	(39%)	Pilar/Neck	19	(61%)	Ant/Inf glenoid	26	(84%)
				Ant/Inf glenoid	19	(61%)	Ant/Inf glenoid	12	(39%)	Pilar/Neck	5	(16%)
External rotation
	Pilar	31	(100%)	Pilar	31	(100%)	Pilar	31	(100%)	Pilar	31	(100%)
Extension
	Pilar	23	(74%)	Pilar	28	(90%)	Pilar	24	(77%)	Spine	18	(58,%)
	Spine	8	(26%)	Spine	3	(10%)	Post/Inf glenoid	5	(16%)	Post/Inf glenoid	8	(26%)
							Spine	2	(7%)	Pilar	5	(16%)
Flexion
	Ant/Sup glenoid	21	(68%)	Ant/Sup glenoid	25	(80%)	Acromion	22	(71%)	Acromion	31	(100%)
	Coracoid	10	(32%)	Coracoid	3	(10%)	Coracoid	9	(29%)
				Acromion	3	(10%)

GT: abutment of the greater tuberosity against the acromion; Lat Hum: abutment of the lateral side of the humerus against the acromion; Ant/Inf glenoid: anterior and inferior part of the glenoid rim; Ant/Sup glenoid: anterior and superior part of the glenoid rim; Ant/Post glenoid: anterior and posterior part of the glenoid rim; Post/Inf glenoid: posterior and inferior part of the glenoid rim.

The glenoid lateralization had also an influence on the localization of the impingement that limited the ROM between the humerus and the pillar of the scapula, especially in extension (p < 0.0001). For adduction and external rotation, the pillar remained the limiting factor of the ROM. In adduction, all impingements occurred with the pillar or the neck irrespective of amplitude. The main differences in the impingement zones occurred in abduction, flexion, and extension (Figure 5).

Figure 5.

Impingements according to the motion in abduction, flexion, and extension. GT: greater tuberosity; Ant/Sup: anterior and superior; Post/Inf: posterior and inferior.

In abduction, abutment of the GT against the acromion was significantly associated with a lower ROM (p < 0.0001). This contact occurred in 92% of the cases with a humeral Onlay design. The greatest abduction motion triggered contact between the acromion and the lateral side of humerus (Figure 6) but an earlier contact could also be observed between the implant PE and the superior part of the glenoid just over the baseplate. It was noted that the GT abutment did not depend on the glenoid lateralization (p = 0.03). In flexion, the motion could initially be limited by the coracoid or the anterior rim of the glenoid and the humerus reached its highest motion when it came into contact with the acromion. In extension, the pillar was the limiting zone with a motion at 14 ± 15° on average. If conflict with the pillar was avoided, a much larger extension could be reached. The impingement with the posterior-inferior glenoid occurred only with the BIO-RSA glenoid (88 ± 9° on average) and a full extension produced conflict with the spine (118 ± 8° on average). The internal and external rotations always developed impingement between the humeral PE and the pillar or the neck of the scapula.

Figure 6.

Simulation of abduction: (a) abutment of the GT was observed in most of the BIO-ONLAY group and (b) the BIO-INLAY group achieved the best abduction by allowing the greater tuberosity to get under the acromion in most of the cases.

Discussion

This study demonstrates the value of the preoperative planning in aiming for improvement in the theoretical ROM. We found a true benefit in glenoid lateralization associated with glenoid neutral inclination and humeral lower inclination especially in rotation. The Inlay design also provided a better ROM in the frontal plane. The use of our preoperative planning software allowed us a strict selection of included shoulders based on version and inclination measurements from an automatically calculated plan. Thus, our initial population was homogenous and allowed avoiding any biases related to the morphology of the glenohumeral joint and the osteophytes. In the second step, we positioned the implant according to a predefined protocol with reproducible rules in order to minimize the confounding factors and to observe only the influence of the studied factors. Finally, the software automatically calculated the ROM, and the results were used to calculate the differences in the ROM between the various plans. We wanted to reflect the evolution in our daily practice comparing the expected results of the prosthesis with a non-lateralized glenoid implant, and their results with a lateralized glenoid positioned at 0° of inclination. Despite the lack of consideration of soft tissues influences (cuff tendon and muscles, capsule) and of the scapulothoracic joint—which preclude us from predicting the clinical ROM, preoperative planning is a means of predicting the maximum glenohumeral ROM and moreover to detect the early impingement.

Lateralization effect on glenohumeral impingement

One of our findings is that glenoid lateralization has a good influence on internal and external rotations, on adduction and on extension by avoiding a contact between the lower part of the PE and the neck of the scapula. Similar results were found in previous analytic studies, in which they observed separately each positioning factor,^12,13 or in cadaveric studies. ¹⁴ Werner et al. ¹³ on a smaller population found the same results and concluded that the lateralization gave better outcomes in rotations. Other authors did similar computer analyses and they also described a decrease in scapular notching and an improvement in the ROM with the glenoid lateralization.^15,16 Then, we assume that the lateralization has a positive effect on the simulated RSA ROM by limiting early impingements. This was also sustained by our gender analysis where motion in rotations, adduction and extension—that generate impingements with the pilar—were different contrary to abduction and flexion that were found similar. Indeed, we assume that an identical lateralization (10 mm) in men and women benefited more to women related to their smaller glenoid size. On the same way, good clinical and radiological results have been already published in the literature especially on the notching: Melis et al. found 88% rate of scapular notching in their series of RSA without glenoid lateralization ¹⁷ whereas Boileau et al. reported a lower rate of scapular notching in the angle-BIORSA series (25%). ³ Athwal et al. also found a significantly lower rate of notching in BIORSA compared with RSA without glenoid lateralization (40% vs. 75%, p = 0.002). ² However, several clinical studies about lateralization are less consensual^18–22 and we suppose that in clinical studies many variations in the surgical techniques and in the choice of the implants or in the graft shape may provide confounding factors and have to be identified.

Glenoid neutral inclination and humeral lower inclination enhances the lateralization effect

An associated factor which reinforced the lateralization effect was the neutral positioning of the baseplate in inclination. In fact, in this study, an objective evaluation of the inclination of the inferior portion of the glenoid with the RSA-angle allowed us to assess the way we positioned the implants over years: positioning the implant directly on the inferior border of the glenoid even with an added 10° inferior reaming induced a remaining superior tilt of the glenosphere that compromised the effect of the lateralization. Thus, it may explain the similar clinical and radiological results achieved by Collin et al. comparing the RSA without glenoid lateralization and the BIORSA ¹⁸ as no modification of the inclination was provided on the glenoid side with the use of a symmetric graft. Based on this observation, we developed the angle-BIORSA that allowed glenoid neutral inclination and lateralization. The validity of this choice seems to be reinforced by the findings of this computational study in which we associated both neutral inclination and lateralization with a gain of +40° for external rotation, +20° for adduction, and +43° for extension whatever was the humerus design. On the humeral side, rotation improvement may also be related to the difference in the inclination of the humeral implant. Previously, Werner et al. analyzed the influence of the inclination on the same Onlay design and found an important effect on the impingement-free adduction, extension, rotation, and global ROM. ¹³ We made the choice not to analyze the effect of humeral inclination on the same humeral implant, as the Inlay or Onlay designs do not have an impact on the rotation and adduction on their own. Indeed, for the rotations and adduction, we assumed that the single cupula position described by the neck shaft angle was critical and the impingement occurred between the cupula and the scapular neck or glenoid.^13,16,23 Moreover, it did not correspond to our practice and to our available implants. Finally, we found that the Onlay design allowed a significantly better motion in rotations and adduction than the Inlay designs due to their difference in inclination.

Improving the abduction with the Inlay design

On the humeral part, the Inlay model provided a smaller offset with a less bulky extramedullary part than the eccentric Onlay model and made it possible for the GT to get under the acromion.

This observation was an interesting finding because we demonstrated that the impingement between the GT and the acromion was associated with a worse abduction and that such a contact had to be avoided. It was noted that glenoid lateralization did not modify the risk of GT contact. The same observations were achieved by Lädermann et al. in an analytic study of the ROM with their shoulder model. ¹² In fact, the authors recognized a major decrease in abduction between the native shoulder (106.6°) and all their prosthetic configurations (which could not exceed 70°). However, in our study, the abduction was significantly higher with the use of an Inlay implant which provided the same abduction as the native shoulder. We assume that the Onlay design has to be reconsidered and that the convertibility of the humerus implant should be weighed in the light of this obvious loss in abduction.

Limits

Our study presents several limitations. We did not compare analytically the variations of the single glenoid inclination, single glenoid lateralization or single humeral inclination for each design separately that could be interpreted as confounding factors, but once again, we wanted to compare our current daily practices and this study was not an analytic study aiming to analyze each factor independently. Given virtual modeling of the ROM, friction phenomena were not identifiable and we could not differentiate between the abutment-impingement type which would be frank abutment, and the friction-impingement type. ²⁴ However, it seemed not be a concern in our study, as the planning objective is to detect the zones where inappropriate contact may occur. Finally, no analysis can be done on the tensioning of the deltoid and on the stresses and shear forces that are applied on the implants. Then, we choose to respect our daily practice to keep the clinical interest of our study and to lateralize either on the glenoid side with no lateralization on the humerus (trail index of 6) or on the humerus (trail index on 12) when no lateralization was applied on the glenoid side, but never on both as it give too much tension on the deltoid. Improvements in the modeling tools such as finite element analysis should give us further data on those points.

Conclusion

Glenoid lateralization delays the glenohumeral impingement in RSA and gives the best ROM in the glenohumeral joint when associated with neutral inclination and humeral 145° inclination especially in internal and external rotation, adduction, and extension. GT abutment has to be avoided to reach the best ROM in abduction and the Inlay design provides the best abduction. The simulation of the RSA ROM on this software is a way (1) to ensure the best clinical ROM for each patient by avoiding early abutment, (2) to limit notching by trying various solutions to minimize impingement, and (3) to facilitate appropriate implant choice with respect to size, design, and the technique to achieve optimal positioning.

ORCID iDs

Marc-Olivier Gauci https://orcid.org/0000-0003-2228-7084

Jean Chaoui https://orcid.org/0000-0002-1245-3096

Adrien Jacquot https://orcid.org/0000-0003-1315-7894