Fig. 18.1
Anteroposterior (AP) view of the pelvis (a) as well as AP view of the left hip (b) and cross-table lateral X-ray of the left hip (c) demonstrate a dislocated left THA with uncemented components. The acetabular component has flipped and migrated. The femoral stem appears to be well fixed
Epidemiology
Aseptic loosening of acetabular and/or femoral components is one of the main complications following THA. An investigation of all revision total hip arthroplasties (THAs) recorded in the Healthcare Cost and Utilization Project Nationwide Inpatient Sample Database found that mechanical loosening was the second most common cause for revision surgery at 19.7% among revision THAs performed between October 1, 2005, and December 31, 2006 [1]. In a separate study that involved two centers over a 6-year period, Ulrich et al. [2] found that aseptic loosening was the most common reason for failure at 51.9%, with the majority developing after 5 years of follow-up.
For cemented THAs, the 25-year survivorship free of revision or removal for aseptic loosening for the primary Charnley THA was found to be 86.5% in one study of 2000 hips [3]. The original Exeter cemented femoral stem (Stryker Howmedica, Kalamazoo, MI) was found to have a survivorship of 93.5% free for aseptic loosening at 33 years of follow-up in a study of 33 hips [4], while the Exeter Universal cemented femoral component (Stryker Howmedica) had a survivorship of 100% free for aseptic loosening at 17 years of follow-up in another series of 325 hips [5]. Hybrid THAs (i.e., those with an uncemented acetabular component and a cemented femoral stem) have also demonstrated satisfactory survivorships [6]. Furthermore, cementation has been associated with an increased survivorship of the femoral stem in patients ≥80 years of age [7].
The use of uncemented components for primary THAs has seen worldwide increase over the last 5–10 years [8]. Several studies have reported excellent long-term survivorship of various uncemented acetabular and femoral components [9–15]. A long-term follow-up of a randomized controlled trial between cemented and uncemented primary THAs revealed that uncemented components demonstrated increased survivorships for all causes as the end point [14]. An analysis of the Nordic Arthroplasty Register Association identified that uncemented implants had a reduced risk of revision for aseptic loosening [16]. However, a review of various worldwide registries found that cemented fixation offered a lower risk of revision in patients older than 75 years [8].
Risk Factors
Several risk factors have been postulated to be associated with aseptic loosening after THA. These can be subcategorized into (1) patient factors, (2) component factors, and (3) surgical technique factors.
Patient Factors
The patient factors that have been determined to date to be potential risk factors for THA aseptic loosening include
Obesity
Bone quality
Activity level
Patient genetics
Obesity has become identified as a risk factor for several THA complications such as infection (superficial and deep) and dislocation [17–19]. The effect of obesity on aseptic loosening is currently controversial. A recent study found that obesity (body mass index [BMI] >30 kg/m2) was associated with early total hip revision for aseptic loosening [20]. The authors found a 4.7 relative risk of THA early revision for aseptic loosening/osteolysis of obese patients compared to non-obese patients. However, a separate recent study that looked at the effects of BMI on the risk of complications and reoperations found that increasing BMI was not a risk factor for mechanical failure or aseptic loosening [17].
Patients with poor bone quality, such as in osteoporosis, have been found to be associated with aseptic loosening of cemented THAs [21]. A study that matched 78 patients with loose cemented THAs to a group of 49 patients with stable implants found that patients with a loose component had significantly lower periprosthetic and lumbar spine bone mineral density [21]. However, male patients have been found to have increased rates of aseptic loosening with the Charnley cemented THA [3, 22–24]. This finding may be secondary to harder bone that leads to a poor cement mantle.
It is currently not clear if activity level is a risk factor for aseptic loosening in THA. However, it is possible that increasing the load or stress on an implant may increase the risk of failure due to loosening. Two relatively recent studies identified increased patient activity level as a risk factor for component loosening [25, 26]. Flugsrud et al. [25] found that men with higher recreational activity levels were at increased risk of acetabular component loosening in a group of cemented and cementless acetabular cups, while Lübbuke et al. [26] identified an increased rate of revision of the femoral component for aseptic loosening in patients with high levels of activity. This association between increased activity and higher aseptic loosening rates could be explained by bearing surface wear.
Lastly, the impact of our genetic makeup has become of interest as a potential cause for early THA failure due to aseptic loosening. The study of single-nucleotide polymorphisms (SNPs) has increased to identify genetic variability that can increase the risk of component aseptic loosening in both cemented and uncemented THAs. Several papers in the literature have identified variation within specific genes that are associated with osteolysis [27, 28]. Furthermore, some studies have found specific polymorphisms that are associated with THA aseptic loosening [29–32].
Component Factors
Advances in THA implant material and design have been devised to promote THA longevity. Both cemented and uncemented components have undergone various modifications over the years to minimize THA failure. There are currently various acetabular cup and femoral stem options from several companies, including cemented and uncemented options, with excellent early to long-term survivorship [5, 10–13]. The main THA component material factor that potentially has played the largest role in improving THA longevity is the conversion of conventional polyethylene to highly cross-linked polyethylene (HXLPE) acetabular liners. One recent study identified significantly more osteolysis in patients that received a THA with conventional polyethylene liners compared to those that received a HXLPE liner at 10 years of follow-up [33]. One of the longest studies to date also found that THA with conventional polyethylene liners had significantly more osteolysis and were considerably more likely to be revised than those with a HXLPE liner at 13 years of follow-up [34].
While the use of metal-on-polyethylene (HXLPE) bearings has been the most widely used articulation, alternate bearing surfaces have also been utilized with the goal of reducing wear rates and improving THA longevity, especially in younger, more active patients. Such bearing surfaces include metal-on-metal, ceramic-on-ceramic, and ceramic-on-polyethylene. These bearings have shown reduced wear rates compared to metal-on-polyethylene; however some may be at a cost, considering the evolving understanding of the effects of metal-on-metal bearings, both locally and systemically [35–38]. Furthermore, the observation of corrosion at the modular head–neck junction leading to “trunnionosis” has prompted some surgeons to utilize a ceramic-on-HXLPE articulation although it is unclear if this will lead to a clinically relevant reduction in wear or failure rates [39–42]. Additionally, one study that compared metal-on-polyethylene (conventional) with ceramic-on-ceramic and metal-on-metal bearing surfaces at 10 years of follow-up found that metal-on-metal articulations had higher aseptic loosening rates than ceramic-on-ceramic bearings [43].
Surgical Technique Factors
Proper surgical technique and component positioning are critical to the early and long-term success of a THA. It is becoming increasingly evident that revision rates are increased with surgeons that have a reduced volume practice [44, 45] or lower surgical skill [46]. A recent study found that patients who received a THA by a surgeon who performed ≤35 primary THAs the year prior were more likely to experience instability or early revision [45]. Component malposition may increase contact stresses and edge loading and in turn may increase wear rates, which can lead to aseptic loosening. For example, a recent long-term study found that the position of a cemented acetabular component at the anatomic hip center for Crowe type-II dysplastic hips resulted in less aseptic loosening rates compared to cups placed at a nonanatomic hip center [47].
For cemented implants, improper cementing technique, particularly of the femoral component, can lead to early loosening and THA failure. First-generation cementing techniques lead to approximately a 30% loosening rate at 10 years [48]. However, the rate of aseptic loosening in cemented femoral stems has significantly decreased with the introduction of enhanced cementing techniques [49].
For uncemented THA components, initial stability is vital to allow for bone ingrowth or ongrowth. Selecting the appropriate implant size is critical to achieve an adequate press fit and initial stability. Therefore, undersizing an uncemented acetabular or femoral component is a risk factor for aseptic loosening.
Prevention
Given that THAs are mechanical devices, complete prevention of aseptic loosening may not be achievable. A certain amount of component wear may be inevitable. However, early loosening can be minimized and the longevity of the implants can be optimized. Aseptic loosening prevention relies on exacting surgical technique combined with optimal patient and implant selection.
Patient weight reduction is becoming an increasingly important part to preoperative patient optimization [50, 51]. Not only does it benefit their overall general health, but it can also decrease complications associated with THA including infection, instability, and early aseptic loosening. Counseling patients on appropriate activities that are compatible with implant longevity may also be helpful. While it is encouraged that patients remain physically active following surgery, they can alter the types of activities, such as avoiding high-impact sports that involve significant running or jumping.
The choice of THA implants can also help to reduce the risk of aseptic loosening. If a polyethylene liner is selected as part of the articulation, then choosing HXLPE will likely improve the THA longevity for younger, more active patients. As previously noted, however, longer term studies are still required to confirm the long-term superiority of HXLPE over conventional polyethylene past 10 years. One recent study found a 100% survivorship for aseptic loosening in young, active patients (≤65 years old) at a minimum of 10 years of follow-up [52]. Alternate bearings may also reduce the risk of wear and loosening. However, they may cause other complications such as adverse local soft-tissue reactions and systemic manifestations from metal ions seen with metal-on-metal bearings or squeaking and ceramic fractures that are associated with ceramic-on-ceramic bearings.
Proper surgical technique can also minimize early implant loosening and increase the durability of the components. Appropriate component positioning of the acetabular cup (version, inclination, and medialization) and femoral stem (version, coronal alignment, and sagittal alignment) is critical to the success of the reconstruction. For cemented components, particularly a cemented femoral stem, proper third- or fourth-generation cement techniques have been shown to improve THA longevity [5, 49]. Contemporary cementing technique relies on appropriate femoral canal preparation with pulsed lavage and drying, placement of a femoral canal restrictor, use of a cement gun, vacuum cement mixing, retrograde canal filling, use of femoral stem centralizers, and most importantly application and maintenance of pressurization during the cementing process and femoral stem placement [53]. Finally, for uncemented components, it is important to obtain implant initial stability by choosing the correct implant size. Acetabular cups may be augmented with screws to further increase stability depending on the surgeon’s preference [54].
Diagnosis
Patient Symptoms and Signs
Aseptic loosening can be a challenging diagnosis to make in some cases or it can be fairly evident as in the case example presented. Evaluation begins with a thorough patient history and physical examination. Patients will typically present with pain in their affected hip. The pain is usually worsened by joint loading with either standing or weight bearing. Patients can also complain of “start-up” pain whereby they report significant pain by changing from a seated to a standing position and taking the first few steps of walking. For aseptic loosening of the acetabular component, pain is typically located deep in the groin area, while femoral component loosening is usually associated with thigh pain. Patients should also be queried for any history of constitutional symptoms that are suggestive of infection, such as fever, chills, or night sweats.
Radiographic Analysis
Radiographic analysis for implant loosening begins with obtaining the appropriate radiographs. These include an anteroposterior (AP) view of the pelvis, and AP view of the affected hip, and a cross-table lateral view of the same hip. It is also helpful to review previous radiographs (if available) to identify any change in radiolucent lines, osteolysis, or component position. It is then important to specifically look at each component in detail on all views to look for evidence of loosening. The radiographic signs of loosening can be quite subtle and may require image magnification to identify a radiolucent line or a comparison between sequential radiographs of specific measurements from the implant and anatomic landmarks such as the greater trochanter.
A radiographic review of the acetabular component requires knowledge of the three DeLee and Charnley zones, which were initially described to demarcate the circumference of cemented cups as seen on the AP hip view [55]. These zones are separated by a vertical line that extends superiorly and a horizontal line that extends medially, both from the center of rotation of the femoral head. Zone 1 corresponds to the superolateral portion of the acetabulum, zone 2 is the medial portion of the acetabulum, and zone 3 is the inferior part. Each zone is assessed for radiolucent lines or areas of osteolysis that can be indicative of loosening. Gross component migration is diagnostic for acetabular cup loosening (Fig. 18.2). While these zones were initially described to assess for loosening of cemented acetabular components, they are also used for uncemented acetabular cups (Fig. 18.3).
Fig. 18.2
Preoperative radiographs of a 64-year-old female with severe rheumatoid arthritis who presented with a failed left THA that was dislocated. Anteroposterior views of the pelvis (a) and left hip (b) along with a cross-table lateral X-ray of the left hip (c) show a cemented acetabular component and a cemented monoblock femoral stem. Her cemented acetabular cup had loosened and migrated superiorly with two broken screws in the ilium
Fig. 18.3
Presenting radiographs of a 56-year-old male patient who presented with left hip pain following a THA that was performed for post-traumatic arthritis at an outside institution. Anteroposterior (AP) view of the pelvis (a), AP radiograph of the left hip (b), and cross-table lateral view of the left hip (c) demonstrate a radiolucent line surrounding the entire circumference of the cup in all three zones suggestive of a loose acetabular component. Note the plates and screws along the posterior column and wall of the acetabulum used for fixation of his original acetabular fracture. The three zones by DeLee and Charnley (a) are depicted by a vertical dotted line and horizontal dotted line that meet at the center of rotation of the femoral head
Radiographic assessment for femoral stem loosening requires a thorough review of the femoral component on all views. Seven zones that surround the femoral component as seen on the AP hip radiograph have been described by Gruen et al. [56] to analyze stem loosening (Fig. 18.4b). These zones were originally described for cemented stems. However, they have been applied to uncemented stems as well. Each zone is reviewed for evidence of radiolucent lines or osteolysis.
Fig. 18.4
Anteroposterior (AP) radiograph of the pelvis (a), AP view of the left hip (b), and cross-table lateral X-ray of the left hip (c) of a 54-year-old male patient who is known for juvenile idiopathic arthritis and underwent bilateral THAs approximately 30 years ago along with a left acetabular component revision in 2012. His radiographs demonstrate a radiolucent line surrounding the entire femoral stem at the implant–cement interface, migration of the femoral component into a varus alignment, and evidence of cement–bone loosening in Gruen zone 1 (b). The patient is also known for bilateral revision total knee arthroplasties, which explains the distal femoral stem in (b) and (c). The seven Gruen zones are also demonstrated around the femoral stem (b)
Certain criteria have been described to determine the likelihood for a cemented stem to be loose. These criteria, described by Harris et al. [57], have been used to determine if a cemented stem is definitely loose (Fig. 18.4), probably loose, or possibly loose (Table 18.1). While these criteria have been used for cemented stem loosening, stem subsidence is an expected evolution of polished, tapered, collarless cemented femoral stems and contributes to force transmission to the femur [58–60]. Therefore, stem subsidence with these stems, which is observed with a radiolucent line at the stem-cement interface in Gruen zone 1 at the superolateral portion of the shoulder of the stem, does not indicate loosening. However, radiolucencies or migration at the cement–bone interface can infer a loose stem.
Probability of stem loosening | Criteria/description |
|---|---|
Definite looseninga | 1. Migration of the femoral stem |
2. Migration of the cement mantle | |
3. Fracture of the femoral stem | |
4. Fracture of the cement mantle | |
Probable loosening | A continuous radiolucent line that surrounds the stem at the cement–bone interface |
Possible loosening | A radiolucent line at the cement–bone interface that surrounds more than 50% of the femoral stem but less than 100% of the stem |
Loosening of uncemented femoral stems (Fig. 18.5) has been predicted using criteria described by Engh et al. [61]. An unstable implant is determined by continued migration or stem subsidence within the femoral canal. Increased cortical density can also be seen at the calcar and at the tip of the stem. Fibrous ingrowth is seen by radio-opaque lines that completely surround the stem without progressive subsidence and stable bone ingrowth defined as the lack of subsidence and lack of radio-opaque lines around the stem.
