Fig. 20.1
AP and lateral radiographs of the right hip and femur demonstrating a fractured modular, bi-body stem with associated periprosthetic femur fracture . Note the severe proximal femoral bone loss
Epidemiology
Total hip arthroplasty (THA) has been shown to be durable , reproducible, and reliable in relieving pain and improving function in patients with hip arthritis [2]. Implant fracture fortunately occurs very infrequently following THA. The Australian Orthopaedic Association National Joint Replacement Registry reported a cumulative revision rate for implant breakage of 2.8% as the principal cause for all revision THAs. The prevalence for each individual component was as follows: (1) implant breakage stem (0.9%); (2) implant breakage acetabular (0.8%); implant breakage acetabular insert (0.8%); and implant breakage head (0.3%) [3]. Other authors have also reported similar rates of failure [4].
Risk Factors
Failures of THA implants are multifactorial and can be due to (1) implant design; (2) implant materials; (3) surgical technique; and (4) patient factors. While no amount of in vitro material testing can predict behaviors of the eventual implant in vivo, certain mechanisms of failures for the various components of the hip prosthesis have been identified.
- 1.
Acetabular component breakage
Breakage of the acetabular component is rare. In the literature, acetabular implant fracture is usually described in cemented all-polyethylene cups with severe wear and osteolysis [5]. Metal acetabular component breakage has also been described in cases with catastrophic wear of the bearing surface and metallosis [6, 7]. Improvements in metallurgy, prosthesis design, and understanding of the importance of polyethylene thickness to long-term survivorship of hip implants have decreased the incidence of acetabular component fractures .
- 2.
Acetabular liner failures
Liner breakage is also relatively rare. Polyethylene liners can fracture or dissociate from the acetabular component [8]. Wear characteristics and acetabular component position can significantly affect reliability and durability of acetabular liners [9]. Suboptimal acetabular component position, thin polyethylene, and large heads can lead to increased edge loading, rapid wear, impingement, and hip instability. Rimmed and lipped liners are particularly susceptible to impingement and fatigue failure [10] (Fig. 20.2).
Fig. 20.2
Fractured polyethylene liner . The outer rims of the liner are most susceptible to repetitive impingement and fatigue fractures
Liner dissociations are either due to failures of the acetabular locking mechanism or failure of the surgeon to fully engage the liner at the time of surgery. While improvements in design have significantly improved the strength and reliability of acetabular component locking mechanisms, incomplete liner seating, impingement, and fatigue failure of the polyethylene liner tabs can still lead to dissociation of the liner from the acetabular component [11, 12] (Fig. 20.3).
Fig. 20.3
(a) AP and lateral radiographs of the left hip demonstrating a dissociated polyethylene liner . (b) Retrieved liner and ceramic ball head. Note the deformation of the liner rim and the metallosis on the femoral head associated with articulation with the underlying acetabular shell
- 3.
Femoral component fractures —Non-modular
Non-modular femoral implant fractures occur in general due to lack of proximal femoral support or impingement. Cemented or uncemented stem breakages were traditionally attributed to lack of proximal implant support in the setting of distal fixation resulting in cantilever bending eventually leading to fatigue failure of the implant [13] (Fig. 20.4). Usually described in extensively coated femoral components, the primary risk factor is the size of the femoral component with stems less than 13.5 mm in diameter being at greatest risk [14]. Other risk factors include nonunion of an extended trochanteric osteotomy and high patient weight [15]. Femoral components can also fail at the neck of the prosthesis due to implant impingement and notching and/or crevice corrosion leading to eventual fracture [16] (Fig. 20.5).
Fig. 20.4
AP and lateral radiographs of the left hip demonstrating a broken fully porous-coated femoral component. The mechanism of failure is due to lack of proximal femoral support leading to cantilever bending and fatigue fracture
Fig. 20.5
(a) AP radiograph of the right hip demonstrating a fracture of the femoral component at the neck of the implant, secondary to impingement with resultant femoral stem notching. (b) Intraoperative image of the broken implant
- 4.
Femoral component fractures—Modular
Femoral components that include a modular neck can break at the neck–body junction (Fig. 20.6) [17, 18]. Modular necks have been made of both titanium alloys and cobalt chromium alloys by various manufacturers. The advantage of a cobalt–chromium alloy neck is that they are stronger and less prone to break. however corrosion is a risk and this failure mechanism has been well described leading to the recall of several implants. Alternatively titanium alloys can be utilized that are not associated with corrosion if the stem is also made of titanium; however the neck is more prone to breakage. The precise mechanism of failure is unknown but there is evidence supporting a fretting fatigue mechanism with subsequent propagation of a bending mechanism leading to failure [19]. The strength of the taper and proper taper engagement and impaction can also significantly affect implant reliability.
Fig. 20.6
Intraoperative image showing breakage of a modular neck femoral component. Specialized instruments or extensile approaches to the femur such as an extended trochanteric osteotomy are necessary for safe implant removal
Stems that include modularity between the proximal and distal segments (so-called bi-body stems typically used for revision procedures) can also break at the modular junction (Fig. 20.1). Breakage at this junction leads to the redesign of several of the first-generation modular, bi-body revision stems that were introduced in the late 1990s and early 2000s. These breakages were similarly associated with excessive body weight and inadequate proximal bone support [20]. Modifications to the design have included enlargement of the taper junction, hardening of the modular taper junction, and improved instrumentation to ensure proper proximal and distal segment taper engagement that all have led to a dramatic decrease in the prevalence of this complication.
- 5.
Ceramic ball head fractures
Utilization of ceramic ball heads has increased due to their favorable wear properties and concerns with corrosion at the head–neck junction when cobalt–chromium alloy heads are utilized [21]. While material and manufacturing improvements have continually decreased the rate of ceramic ball head fractures over the past 15 years, occasionally failures can still occur. Most fractures occur within the first 5 years following implantation and alumina matrix composite ceramic heads are less likely to fracture compared to pure alumina ball heads . Additionally, a 28 mm ball head is more likely to fracture compared to larger ceramic ball head sizes. Finally, and most importantly, taper design, taper contamination, and proper impaction significantly affect the fracture risk for ceramic ball heads [22, 23].
- 6.
Ceramic liner fractures
Ceramic liner fractures have also decreased in frequency over the same time period but are presently more common in contemporary practice when compared to ceramic femoral head fractures. Elimination of lipped liner and sandwich-type ceramic insert designs have led to the overall reduction in ceramic liner fractures [24]. Modern ceramic liner fractures most frequently occur within the first 24 months following implantation: pointing to surgeon and implant-related factors as the underlying causes for failure [25]. Risk factors for ceramic liner breakage include (1) poor component position, (2) incomplete seating of the ceramic implant, (3) liner chipping during insertion, and (4) hip instability [26].
Prevention
The causes of implant breakage following THA are multifactorial. Unanticipated consequences of design, materials, and modularity have contributed to the fractures of certain implant designs. However, proper planning, good surgical technique, and adherence to sound principles of hip reconstruction can minimize the risk of implant failure in hip arthroplasty.
The process begins with proper preoperative planning and careful implant selection. In most instances, the degree of bone loss or deformity will dictate the type of implant required for fixation and reconstruction. In cases where the native medullary canal is extremely narrow, avoidance of a long, small monoblock extensively coated femoral component may be prudent. Furthermore, if a smaller (less than 13.5 mm) fully porous-coated femoral component is used in conjunction with an extended trochanteric osteotomy, strut augmentation of the osteotomy site may impart additional proximal support and stability [15]. Modularity should be used only when necessary in order to eliminate another potential interface for failure. Finally, selection of implants with good track records can reduce risks for implant breakage following hip arthroplasty.
Good surgical technique and adherence to sound principles of hip reconstruction are critical to minimize postoperative complications. Proper acetabular component position can minimize wear, impingement, and instability. Additionally, adequate surgical exposure and circumferential engagement of the locking mechanism can minimize liner-related complications. This is particularly true for ceramic liners. Most modern liner fractures are due to incomplete seating and chipping of the ceramic liner during implantation [27]. Proper taper assembly and femoral head impaction can minimize the risk of ceramic head breakage.
Diagnosis
Diagnosis for the broken hip implant can range from the subtle to the obvious. Depending on the failed implant, a patient’s complaint can range from noise and mild discomfort to severe pain and inability to walk. A detailed history and physical exam form the foundation of the workup for a painful THA. Characteristics of symptoms such as onset, timing, and events that exacerbate pain and symptoms can give clues to the type of failure. Additionally, any prior history of trauma, infection, and rest pain should be elucidated. While most implant breakages are easily visualized on plain X-rays, in some cases the findings can be quite subtle (Fig. 20.7a, b). Similarly, breakage of a ceramic liner can be difficult to visualize on plain X-rays and the surgeon should have a low threshold to get a CT scan to identify liner breakage if the patient presents with new onset of pain, crepitation, or noise from the hip.
