or Pegged Polyethylene Glenoid Components

Fig. 8.1

Metal-backed glenoid component

The most common complication of TSA is the failure of the polyethylene glenoid component which accounts for the majority of the deleterious outcomes and manifests clinically by pain, loss of function, and presence of a clunking noise [7–10]. While many factors have been described as possible contributors to glenoid component failure, a systematic understanding of these factors is lacking. The main problem seems to be the loosening and the following revision which is troublesome for surgeons because of the restricted amount of bone reserve of glenoid. High rates of radiolucency at the bone-cement interface were previously reported numbers of times in the literature (Fig. 8.2). However, this fact does not always show up clinically as symptomatic loosening during the postoperative follow-ups. Nevertheless, with the better radiological and clinical survival periods in years, radiolucency may become a more important radiological finding of impending glenoid component failures than it is today.

../images/429740_1_En_8_Chapter/429740_1_En_8_Fig2_HTML.jpg — Fig. 8.2

The bone-cement interface dissociation

The bone-cement interface dissociation (Fig. 8.2) is the major problem encountered at follow-ups which can also be more frequent by the admixture of antibiotics and by preparation methods that introduce porosity into cement. The interposing cement in the bone-cement interface brings out the risk for dissociation because of a thin layer of cement which is brittle and highly susceptible to fatigue, fracture, and displacement [11]. Additionally, an inadequate interposition of the cement results in loss of seating and loss of grouting effect of the cement which causes poor support for the component [12]. The mechanism of glenoid loosening is thought to be repetitive and eccentric loading of the humeral head on the glenoid, the so-called “the rocking horse” phenomenon. This eccentric or edge loading condition produces a torque on the fixation surface that induces a tensile stress at the bone-implant or bone-cement-implant interface, potentially causing interfacial failure and glenoid dissociation [13]. Actually, physiologic bone-cement interface stress can easily exceed the stress needed to initiate cement mantle cracks. A widely accepted common value for crack initiation in polymethyl methacrylate (PMMA) is about 5–7 MPa, and finite element models report this value with a lowest maximum principal stress (tensile) of approximately 6 MPa for the keeled design and unloaded arm [14, 15]. Therefore, it is obvious that there is an unavoidable risk for bone-cement interface dissociation. This prediction of failure risk is even more likely to be true when one considers that the glenohumeral loading used in the aforementioned study was in an unloaded and soft tissue-free arm [15].

The incidence of glenoid loosening was reported previously by a number of studies as low as 0% to as high as 96%, assuming the radiolucent lines as indicators of early loosening [7, 16–21]. These rates are unacceptable for the survival of a prosthesis to lead revision procedures. The authors dedicated in shoulder surgery investigated broadly the mechanisms of loosening to find a solution. Previously, Karduna et al. have found that the humeral head translates 1.5 mm in the anterior-posterior (AP) direction and 1.1 mm in the superior-inferior (SI) direction during the active glenohumeral motion including rotation and translation [22]. Similarly, McPherson et al. reported 4 mm translation of humeral head during the active motion of the glenohumeral joint in healthy individuals which they measured from the radiographs [23]. This physiologic motion is thought to be resulting from the bony incongruence of the glenoid and the humeral head and the congruence of the surrounding soft tissue, the articular cartilage, and the labrum. However, when the glenoid is resurfaced with a human-made implant, physiologic translation of the humeral head turns into eccentric loading of the bone-cement or bone-metal backed component because of the inadequacy of UHMWPE to mimic the viscoelastic properties of the articular cartilage and labrum [13]. This eccentric loading leads to rocking forces at the glenoid component which is shown to be increasing with the early radiolucencies at the subchondral bone-cement interface caused by incomplete glenoid component seating [17, 24]. Therefore, recent biomechanical, animal, and retrospective studies have involved mostly the glenoid design, rather than traditionally accepted cementing technique or thermal necrosis, in the development of the glenoid lucency [16, 25, 26].

So many methods have been advocated to improve fixation and long-term stability of the glenoid component previously. These include preservation of the subchondral plate, concentric spherical reaming of the glenoid cavity, enhanced biomaterials, mismatching of the diameters of the glenoid and humeral head, patient-specific components, cementing techniques, and new glenoid designs [27–32]. In this chapter, the review of the literature will be over the pegged and keeled glenoid components (Fig. 8.3).

../images/429740_1_En_8_Chapter/429740_1_En_8_Fig3_HTML.jpg — Fig. 8.3

Pegged and keeled glenoid component designs of total shoulder arthroplasty

With the presentation of a comparison between cemented pegged glenoid component and conventional keeled components in a well-designed finite element model by Lacroix et al. in 2000, the glenoid component-glenoid bone interface stresses were better understood and the hope for better longevity in TSA was raised again [15]. In that study, the authors used an advanced finite element model prepared wisely with quantitative computerised tomography (CT) of a normal scapula to investigate the stress of cemented glenoid component fixation and to quantify the probability of cement failure and found that keeled designs were more preferable over pegged designs in patients with rheumatoid arthritis while vice versa was valid for the normal bone. Actually, their study was planned and carried out comprehensively. The twisting of the components, which is described as the moment about the centre anchorage system of the component, was better resisted by the keeled designs than the pegged ones. However, the cement stress was 10% at the cement mantle above 5 MPa with the keeled design, compared to less than 1% with the pegged design, which was reversing in rheumatoid arthritic bone. Therefore, the twisting forces seem to be not playing a crucial role in loosening.

In 2002, Lazarus et al. reviewed retrospectively the postoperative radiographs of 39 patients with keeled glenoid component and 289 with pegged glenoid components. They used the Franklin method (Table 8.1) to grade the degree of radiolucency around the keeled glenoid components and modified Franklin method (Table 8.2) for the pegged glenoid components [33]. They found that the pegged components had better cementing than keeled components. They also assessed the seating profiles with a new scale described firstly in their paper reflecting the amount of host subchondral bone directly in contact with the back of the glenoid component (Table 8.3) and found that incomplete seating was more common with the keeled components. The patients were part of a multicentre study in which 17 surgeons from 17 different centres participated. The trademark for the prosthesis was same in all patients (Global Total Shoulder System, DePuy Orthopaedics, Incorporated, Warsaw, Indiana), but that of the methyl methacrylate is not mentioned in the study.

Table 8.1

Franklin method of grading scale for radiolucencies about keeled glenoid components

Grade	Finding
0	No radiolucency
1	Radiolucency at superior and/or inferior flange
2	Incomplete radiolucency at keel
3	Complete radiolucency (≤2 mm wide) around keel
4	Complete radiolucency (>2 mm wide) around keel
5	Gross loosening

Table 8.2

Modified Franklin method for grading scale for radiolucencies about pegged glenoid components

Grade	Finding
0	No radiolucency
1	Incomplete radiolucency around one or two pegs
2	Complete radiolucency (≤2 mm wide) around one peg only, with or without incomplete radiolucency around one other peg
3	Complete radiolucency (≤2 mm wide) around two or more pegs
4	Complete radiolucency (>2 mm wide) around two or more pegs
5	Gross loosening

Table 8.3

Grading scale for completeness of glenoid component seating

Grade	Finding
A	Complete component seating
B	<25% incomplete contact, single radiograph
C	25–50% incomplete contact, single radiograph
D	<50% incomplete contact, both radiographs
E	>50% incomplete contact, single radiograph

Trail and Nuttall compared patients with pegged and keeled components in their study with a mean follow-up of 5.7 years and found that 90% of keeled components had a radiolucent line of more than 1 mm in at least one zone while 36% of pegged components had that. The incidence of radiological evidence of loosening was 53% although none required revision, and it was significantly less in the pegged designs [34].

Gartsman et al. compared 23 patients with keeled components and 20 with pegged components by scaling the radiolucencies on the early postoperative radiographs obtained within 6 weeks of surgery which were evaluated by three raters. The rate of lucency was 39% in the keeled components, which was significantly higher than the rate of 5% observed in the pegged components [35].

Roche et al. reported in 2006 that the keeled and pegged designs revealed no significant difference in edge displacement occurred before or after cyclic and eccentric loading [36]. They also added that each keeled and pegged designs remained fixed firmly following the tests showing the trustable resistance to edge displacement.

Nuttall et al. presented the results of their study in 2007 analysing the radiostereometric properties of the total shoulder arthroplasty patients, 10 with keeled and 10 with pegged glenoid components. In that study, the relative movement of the glenoid component with respect to the scapula was measured over a 24-month period, and the highest maximum total point movement was found to be 2.57 mm for keeled and 1.64 for pegged eroded components [37]. Whereas all components had moved at last follow-up, keeled components revealed significantly greater migration than the pegged ones. There was no significant difference between the designs in terms of pain relief and functional improvement at the end of the 24-month follow-up in that study; however, this finding does not change the fact that it can develop in a long-term follow-up. Rahme et al. presented in 2009 their findings which revealed no significant difference at the end of 24-month follow-up between keeled and in-line pegged glenoid components in terms of Constant-Murley score improvement, average micro-migration, and translation or rotation of the glenoid components with radiostereometric analyses [38].

In a study made by Edwards et al. in 2010, whether the difference between keeled and pegged glenoids in terms of postoperative radiolucent lines were decreased with the modern cementing techniques including the systematically and step-by-step application of saline solution lavage, sponge drying, and cementing under pressure using catheter tip syringe with no cement at the back of the glenoid component-glenoid bone interface was investigated [39]. The rate of glenoid lucency between pegged (0%) and keeled (15%) components did not differ significantly on immediate radiographs. However, after an average of a 26-month period, the higher risk in keeled components was obvious (15% versus 46%).

Throckmorton et al. also pronounced the similar outcomes with pegged and keeled designs in terms of pain relief, functional improvement, and risk for loosening at the end of a mean follow-up of 51.3 months for the keeled group and 45.7 months for the pegged group [12]. They reported early radiolucency on radiographs taken immediately after operation only in 4 of 50 pegged components and 1 of 50 keeled components. The decrease in radiolucent lines on early postoperative radiographs might be due to the modern cementing techniques which achieve a low incidence of early radiolucent lines at both the bone-cement (fixation) interface and the subchondral bone-component (seating) interface [40].

Raiss et al. reported in 2011 the outcomes of their fresh frozen cadaver study with ten pairs of scapulas [41]. There was a strong negative correlation with the bone mineral density (BMD) and the cement penetration in both type of designs, and the penetration amount was significantly higher in pegged components. However, the mean pull-out strength was significantly higher in the keeled group (1093 N) than the pegged group (884 N). Since the loosening of the glenoid components is thought to be secondary to the eccentric loads and rocking effect of the humeral head over the glenoid surface [13], the high pull-out strength did not affect the clinical and radiographic outcomes.

There are also different types of keeled glenoid components such as with anterior offset or inferior offset presented in the literature aiming to provide decreased stresses at the bone-cement interface and similar outcomes with pegged components. Murphy et al. investigated the anteriorly offset keel which preserved a greater amount of the more dense bone, which is mostly placed at posterior glenoid, and associated with lower stress because of the more directly aligned with the applied force [42]. Similarly, Orr et al. reported the inferiorly offset keel and pronounced that better replication of normal stresses of the glenoid can be achieved when compared to centrally located keels [43]. However, these two finite element models do not include comparison with pegged components. The main designs of the pegged glenoid components are as follows: the in-line pegged design in which three pegs of the component are arranged in a row and the outline design in which the pegs are dispersed behind the polyethylene back (Fig. 8.4). The advantage of the in-line peg (Fig. 8.5) design is to occupy less cavity of the glenoid bone than the outline design, thus preventing posterior penetration which simulates the keeled design preponderating in defective glenoids.