Implant Options



Implant Options


Kenneth J. Faber, MD, MHPE

G. Daniel Langohr, PhD





HUMERAL STEM OPTIONS

Humeral components have undergone profound changes over the past 4 decades. Stem fixation techniques have improved, and implant size has steadily diminished to reduce bone resection, preserve host bone, simplify the treatment of proximal humeral deformity, and minimize stress shielding.


Humeral Implant Materials

Titanium is the most commonly used material for the humeral stem. This material is relatively inert, has excellent biocompatibility, and a modulus of elasticity resembling normal bone that can promote ingrowth to create a stable bone-implant interface. Cobalt chrome (CoCr) has a modulus of elasticity that is considerably greater than titanium and is a material that is infrequently used for humeral stems.6


Fixation Options: Cemented

Shoulder cementing techniques have benefitted from improvements and advances in hip and knee arthroplasty cementation (FIGURE 12.1). Current cementing techniques include the use of cement restrictors, canal cleansing, and cement pressurization.

There are several advantages to cement fixation. Cement provides immediate early stem fixation and avoids the risks of stem subsidence. The stem size can be reduced to facilitate accurate positioning within the canal that optimizes head coverage of the humerus resection. Cemented stems have an excellent performance record with low rates of aseptic stem loosening.7,8,9,10

There are several disadvantages to cement fixation. Cement can extrude from the intramedullary canal through nutrient artery fenestrations and compromise the radial nerve.11 Well-fixed cemented stems are often challenging to remove, and complications such as iatrogenic fracture and thermal injury to bone and neurological structures have been reported.12 Occasionally, an intentional humeral osteotomy is required for stem removal, and the subsequent reconstruction may require bone grafting and supplemental fixation that can delay postoperative rehabilitation.13,14,15


Fixation Options: Cementless

The transition to cementless (or uncemented) stem fixation has been motivated by the technical challenges associated with the extraction and revision of cemented stems and the increasing ease and familiarity with the use of cementless devices.16,17 Cementless humeral stems can be broadly categorized as metaphyseal or diaphyseal filling implants (FIGURE 12.2). Metaphyseal filling stems rely on proximal bone ingrowth/ongrowth to a textured surface. Initial proximal stability can be enhanced by the addition of cancellous autograft harvested from the resected humeral head.18 Diaphyseal filling stems are dependent on endosteal fixation adjacent to the metaphysis.

Uncemented stems have several advantages. Humerus preparation and prosthesis implantation are simplified and less time-consuming than implantation of cemented stems. Shortened case times and the avoidance of disposable canal preparation equipment such as pulse lavage, restrictors, and cement may provide increased value.

The disadvantages of cementless fixation include iatrogenic fracture, incomplete implant seating, implant migration, and stress shielding. Iatrogenic fracture can occur during canal reaming, broaching,
or implantation. Stable fractures can be treated with a modified rehabilitation program or cerclage wire fixation. Unstable fractures can be converted to a smaller cemented stem with cerclage wire fixation. Incomplete implant seating can alter joint loading, rotator cuff tensioning and cause persistent postoperative pain requiring revision (FIGURE 12.3).19
















Similarly, implant migration can alter normal glenohumeral alignment resulting in pain or instability (FIGURE 12.4).













CONVERTIBLE STEMS

Common reasons for clinical failure of anatomic TSA include progressive rotator cuff disease, glenoid component loosening, and shoulder instability. In each of these instances, the humeral stem may be well fixed and well aligned. The development of convertible platform systems specifically addresses this clinical scenario and permits revision to a reverse prosthesis without removal of the stem. Clinical studies of platform systems indicate that fewer complications occur and blood loss is diminished when compared to revisions requiring stem removal, but there may be a risk of excessive humeral lengthening.20,21


Humeral Stem Options: Length, Geometry, and Surface Characteristics

Humeral stem vary in length (FIGURE 12.5), ranging from standard-length stems having an overall length between 100 to 150 mm, short stems having overall lengths between 50 to 100 mm, to stemless designs which have lengths of less than 50 mm.22

Stem width also varies and is commonly classified in terms of filling ratio (FIGURE 12.6), which is the size of the stem relative to the diameter of the humeral bone.23 Generally, a larger filling ratio is associated with more bone removal and may contribute to humerus stress shielding. The presence (or absence) of a proximal collar that provides initial stability and transfers implant loading to the proximal cortex is also a common variant in humeral stem design.

Humeral stem geometry and design are both major factors in determining the type of humeral fixation achieved, which can be metaphyseal (proximal), diaphyseal (distal), or some combination of both.













Standard Stem

Standard-length stems include a fixation structure that extends distally from the humeral resection plane into the humeral canal (FIGURES 12.5 and 12.7). The surface characteristics of the standard stem are dependent on whether it is cemented or uncemented. Cemented variants typically have a constant polished or slightly roughened surface finish that provides an interface for adhesion between the stem and cement. Uncemented stems have a metaphyseal surface that is treated to create a rough and/or porous surface to promote proximal bony ingrowth, while the distal aspect of the stem is commonly polished to prevent distal fixation. Some manufacturers have dedicated stems for cemented and uncemented applications, while others have a universal stem with uncemented surface characteristics that is intended for both fixation techniques. The filling ratio is typically larger proximally and smaller distally for metaphyseal fixation stems, while the opposite occurs with diaphyseal fixation stems.






As a result of the high prevalence of stress shielding with standard uncemented stems, shorter humeral stems have been introduced.


Short Stem

Short-stem humeral components maintain the proximal metaphyseal contact surfaces, but the distal stem is either absent or greatly reduced (FIGURES 12.5 and 12.8). Short-stem humeral components are also available
for cemented and uncemented fixation, with the former typically exhibiting constant smooth polished or roughened contact surfaces, and the latter employing various forms of spatially varying rough and/or porous surfaces to promote bone ingrowth for fixation. Like standard stems, the filling ratio of short stems typically decreases moving distally.






To further address the concern of humeral stress shielding, as well as to increase bone preservation, stemless humeral fixation components have most recently been developed.


Stemless

Stemless humeral fixation components incorporate fixation structures, which interface only with the most proximal humeral bone (FIGURES 12.5 and 12.9) and include two classifications of fixation; central and peripheral fixation. Central fixation engages the central aspect of the humeral trabecular bone beneath the humeral resection plane, whereas peripheral fixation involves the interaction of the peripheral region of humeral trabecular bone closer to the cortex. The amount of load transfer at the proximal cortex of the humeral resection plane also plays a role in determining the load transfer from the implant to the surrounding bone.

Stemless humeral reconstruction allows accurate positioning of the humeral head on the resection surface without referencing the distal canal. This is particularly helpful when treating arthritis that is associated with proximal humeral deformity (FIGURE 12.9). In addition, failed stemless components can often be removed without damaging the proximal humerus and revised to conventional components (FIGURE 12.10). Early clinical results with stemless devices are encouraging, and this may be partially due to the more anatomic positioning of the humeral head, although the long-term clinical outcomes are still not known.22


Resurfacing

Humeral resurfacing prostheses are implanted after removal of the remaining articular cartilage and subchondral bone. The humeral head cancellous bone is preserved, and fixation is achieved with cancellous bone ingrowth to the hydroxyapatite or plasma sprayed back surface of the component and the central alignment post (FIGURES 12.5 and 12.11). Conservation of the cancellous bone makes glenoid exposure more difficult, and resurfacing devices are being gradually displaced by stemless devices.7


HUMERAL STRESS SHIELDING

Stress shielding, which manifests itself as a reduction in proximal bone following humeral reconstruction, occurs as a result of bone remodeling in response to applied stress according to Wolff Law. In the case of humeral reconstruction, this occurs due to the reduction in loading of the proximal humerus as a result of the insertion of the humeral stem, which now carries a proportion of the joint load distally. The first clinical reports of stress shielding were based on observations of extensively coated implants that were intended to achieve metaphyseal and diaphyseal fixation23,24 (FIGURE 12.12). In some cases, the stress shielding was profound and mimicked observations of extensively coated uncemented hip arthroplasty stems.25,26







Standard stems are at risk of facilitating stress shielding if the distal portion of the stem is well-fixed in the diaphysis. Clinical reports of a variety of standard stem humeral implants have found the prevalence of stress shielding in uncemented stems to range from 5% to 63% of patients.23,27,28 Larger filling ratios have also been correlated with increased remodeling and proximal bone resorption.23,29






In the case of short stems, the elimination of the distal stem theoretically reduces the risk of distal fixation and stress shielding. Computational studies have shown that as humeral stem length is reduced, so does the
corresponding change in humeral bone stress compared to native bone, meaning that shorter stems have the potential to reduce the risk of proximal humeral stress shielding.30 However, clinical reports have shown stress shielding still occurs with short stem humeral components ranging in prevalence from 14% to 52%31,32 and radiolucencies or bony adaptations present in 71% to 80% of patients33,34 (FIGURE 12.13).






It has also been reported that up to 9% to 17% have been found to be either loose or at risk of loosening,33,35 although the presence of proximal ingrowth coating has been shown to reduce the risk of loosening significantly.36 Although short stems still can result in humeral stress shielding, they are better than standard stems, with comparative clinical results showing less prevalence of cortical thinning (50% vs 74%) and partial calcar osteolysis (23% vs 31%).37

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Jun 23, 2022 | Posted by in ORTHOPEDIC | Comments Off on Implant Options
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