Instability in Total Hip Arthroplasty

Abstract

Total hip arthroplasty (THA) is becoming one of the most frequently sought-after surgeries in orthopedics. As the techniques and implants continue to evolve, the stability of the prosthesis is always at the forefront of the surgeon’s mind. Multiple factors contribute to implant stability and there are many intraoperative decisions that can be made by the surgeon to increase stability. Techniques including approaches, adjusting length, adjusting offset, as well as implant choices can dictate stability in THA. There are multiple options that exist including different liners and constraint. One non modifiable variable which surgeons often struggle with is the spinopelvic relationship which can also affect stability post operatively. These factors include lumbar arthritis, variable pelvic tilt, and others that can make a routine approach to a total hip unsuccessful and increase the risk of post-operative complications. Ultimately there are many things to consider when approaching THA in patients, especially in the setting of abnormal pathology.

1. Introduction

Hip arthroplasty remains one of the most successful surgeries offered today; however, with a prosthetic hip component, a unique possibility of dislocation arises [1]. The incidence of instability within revision hip arthroplasty is estimated from 17 to 25% with a mean cost of care at $77.851.24 [2, 3]. With rates rates of primary hip arthroplasty increasing, the projected financial burden on the healthcare system remains large [4, 5, 6]. Historically, implant designs featured smaller femoral head articulations, such as the 22 mm femoral heads of the Charnley hip, which were associated with instability rates as high as 4.8% [7, 8]. Since the landmark Morrey article in 1982, improved implant designs and surgical techniques have evolved lowering the dislocation rate from 3.2% to less than 2% [9, 10]. Although a large percentage of instability can be managed nonoperatively, instability remains the most common indications for revision arthroplasty within the United States [4, 11]. Numerous risk factors exist including patient demographic variables, approach, surgeon learning curve, spinopelvic relationship, and indication for surgery. These factors should be taken into consideration when projecting a specific patients’ potential risk of subsequent instability.

A stable THA relies on understanding the biomechanics of femoral head size and center of rotation (COR) [12]. Briefly, hip offset is defined as the linear distance from the femoral COR to the axis of the femoral shaft. A medial shift in the center of rotation decreases the moment arm of the abductors, thereby changing abductor tension and increasing potential risk of instability (Figure 1). Conversely, an increase in femoral offset adds to abductor tension and reduces potential instability [14]. Hip stability can also be achieved through modulation of femoral head size and consequently jump distance. The linear distance required for the femoral head to travel prior to dislocation is directly proportional to the femoral head size. By increasing the size of the femoral head a larger displacement is required prior to dislocation (Figure 2). Surgical manipulation of hip anatomy through biomechanics is central towards optimizing patient stability.

Figure 1.

Femoral offset and subsequent abductor moment arm [13].

Figure 2.

Large diameter femoral heads have larger jump distances than smaller diameter heads [14]. Of note, due to the fixed radius of the acetabular component, a larger femoral head will decrease the space available for polyethylene. This can be seen in Figure 2A versus Figure 2B.

2. Risk factors

Several modifiable and nonmodifiable risk factors should be considered prior to undertaking hip arthroplasty. Modifiable risk factors include tobacco, alcohol use, and obesity [15, 16]. Modern hip arthroplasty utilizing press fit implants relies on immediate stability at the bone/implant interface through a variety of tapers and coatings. Although the exact mechanism is not understood, this stability is weakened from tobacco use. Several researchers have demonstrated the adverse effects of delayed bone healing with tobacco use and this theory has been extended to include delayed bone-implant ingrowth [17, 18]. Elective arthroplasty offers a unique opportunity for patients to cease smoking, and some literature demonstrates continued abstinence [19]. A careful history with a targeted effort at reducing modifiable risk factors should be considered prior to hip arthroplasty.

Tobacco and alcohol use are correlated with wound complications and potential instability [16, 17]. Also, patients who abuse alcohol are less likely adhere to precautions and suffer more frequent falls, leading to interprosthetic instability. The immunosuppression from alcohol misuse has shown an increased risk of prosthetic joint infections thereby impairing bony ingrowth [17].

Currently, more than two-thirds of Americans are classified as obese (body mass index (BMI) ≥ 30 kg/m2) [20]. Groups with the highest BMI are increasing in size at the fastest rate, as evidenced by the greater than 50% annual increase in prevalence of patients with a BMI ≥ 40 kg/m² [21, 22]. Elevated BMI will increase the soft tissue envelope around the hip, thereby increasing the risk of implant malpositioning. This malpositioning along with soft tissue impingement are known risk factors for instability [23]. Patients with elevated BMI tend to be younger. Younger aged patients statistically place more stress on their implants and with the increased weight these patients may have elevated wear rates and higher risk of aseptic loosening [24].

Non modifiable risk factors include advanced age, cognitive impairment, and in some earlier studies, female sex [16]. Several comorbid conditions also predisposing patients to dislocation following THA include developmental dysplasia of the hip, neuromuscular disorders, and other connective tissue disorders. Abductor muscle deficiency, prior surgical revision, a history of instability, and prior spinal disease or surgery [25, 26, 27, 28]. Previous instances of instability are also risk factors for future instability events.

3. Preoperative optimization of hip stability

Due to the numerous risk factors for instability, a thorough preoperative evaluation and identification of appropriate surgical approach should be performed. Several hip arthroplasty approaches exist with varied evidence on the risk of subsequent instability. Historically, the posterior approach was associated with highest rates of instability [29, 30]. This was reinforced with a subsequent large volume Kaiser series that demonstrated improved stability with the direct anterior approach over traditional posterior approaches [31]. However, recent literature has brought this into question, largely demonstrating that with capsular repair the posterior approach is no superior to alternate approaches [32, 33, 34]. Critics highlight the selection bias of these capsular repair/posterior approach papers stating they reflect academic practices and do not adequately reflect the community [35]. Further controversy exists when analyzing large joint registry databases. Both the Australian (122,345 primary THA) and Dutch (166,231 primary THA) Registries demonstrate a reduced risk of instability with the anterior approach [35, 36]. From a revision perspective, it appears that changing approach does not affect overall rate of instability [37]. Ultimately it is recommended that surgical approach be utilized at the discretion and comfort of the surgeon with the recognition that the anterior approach may have improved stability. If the posterior approach is preferred then careful capsular closure should be performed [38, 39].

Preoperative optimization of body mass index (BMI) continues to be an ongoing debate. Multiple studies demonstrate a slight preponderance for instability in cohorts with heavier BMI—with 5% increased risk for each BMI unit exceeding 35 kg/m² [40, 41]. Although the exact etiology of instability in heavier patients remains unknown, possibly resulting from combinations of deeper surgical field causing implant malposition versus muscular weakness; nonpharmacologic weight loss does seem to work at reducing BMI in some patients [42, 43]. From the perspective of instability, it remains unknown if weight loss causes a clinically significant risk reduction in postoperative instability; however, the generalized physical and mental health benefits certainly warrant an attempt at reducing BMI [44].

4. Intraoperative optimization of instability

4.1 Introductory statement

Surgeons should be aware of the impact their approach, implants, and implant positioning has on patient outcomes. When performing an arthroplasty for fracture, a hemiarthroplasty can be an option in a less active patient but this does not confer the longevity that a total hip offers. Studies note less morbidity, decreased operative time and decreased blood loss with a hemi versus total [45]. If the implant of choice is a hemiarthroplasty, a decision between a unipolar and a bipolar implant must be made. Proponents of a unipolar arthroplasty state that the hip stability primarily comes from the larger femoral head component and the dual articulation of the bipolar component provides negligible stability [46]. Yang et al. published a systematic review illustrating statistically significant decrease of cost and increased acetabular erosion with a unipolar arthroplasty [47].

Treatment of displaced femoral neck fractures in the elderly population continues to be a subject of controversy in recent literature. The New England Journal of Medicine recently published a study which included 1498 patients, ages 50 or older, across 80 centers, in 10 countries where patients were randomly assigned to either hemi or total hip arthroplasty group following a displaced femoral neck fracture. Despite only having a 2 year follow up, this cohort exhibited no significant difference in secondary procedures performed [48]. Some have critiqued this study questioning if a 2 year follow up was sufficient time to detect a difference. Another study noted no difference of revision rates at 5 years but improved quality of life favoring the THA cohort and reduced surgical time favoring the hemi arthroplasty cohort [49].

4.2 Femoral component: version + proper tensioning

Studies have shown large inconsistencies of the proximal femur from patient to patient. A patient’s proximal femur may undergo morphological changes throughout their lifetime due to osteoporosis and age [50, 51]. Manufactures have designed the femoral component with variability to accommodate the irregularity of the native proximal femur [52, 53]. Hip dysplasia, while relatively common, can add up to 60 degrees variability in proximal femoral morphology and contract stress [54, 55].

Femoral anteversion is the angle between an axially projected line along the femoral neck and posterior condyles (Figure 3) [56]. This anteversion is essentially the extent the implants are “pointed “ventrally and has wide variation [57]. This angle can be measured preoperatively via CT scan or with a variety of different x-rays [58, 59]. Combined implant anteversion was popularized to improve hip stability and decrease intraprosthetic impingement while providing a functional range of motion for the patient [60]. Some surgeons have become proponents of “femur first” preparation since noncemented implants are constrained by the proximal femoral anatomy [61]. Once the femoral anteversion is measured, appropriate anteversion can then be “dialed into “the acetabular component as this is easier to change.

Figure 3.

Measurement of femoral Anteversion (FNA—Femoral neck angle) [56].

Femoral length is a measurement from the acetabular teardrop to the proximal femur [62]. This change is often easily noticed by patients and can often cause discomfort after a total hip replacement [63, 64]. Femoral length can be changed independently by changing the position of the implant within the femoral intermedullary canal or changed in conjunction with offset when the surgeon changes the femoral neck component.

Femoral offset is a measurement of the distance from the femoral intermedullary canal to the center of rotation [12]. An increase in this measurement will allow the femur to sit further away from midline, thereby increasing the abductor lever arm [65]. Both femoral length and femoral offset will increase tension on the gluteal musculature and provide stability. The surgeon must take caution and balance hip stability with an increase in offset as excessive femoral offset will cause pain to the gluteal musculature, increase implant micromotion and overload the femoral component [66, 67, 68]. Femoral implants have different ways to increase offset. Some implants change offset based on a “medial shift” of the stem/neck takeoff while other implants change the neck shaft angle. The average neck/shaft angle in the Caucasian population is 130 degrees and by changing offset in different ways, the surgeon may restore “normal patient anatomy” and properly tension the soft tissues [52, 53]. It is important that the surgeon becomes familiar with how changes in the neck/shaft angle will change femoral neck length (Figures 4 and 5). In certain situations where a large femoral neck is needed a skirt may be required. This skirt is needed to properly engage the morse taper while moving the center of rotation away from the stem base. The “skirt” on the neck becomes necessary at differing + neck options based on manufacture designs and will vary based on skirt length and thickness. These femoral neck skirts decrease the head to neck ratio thereby possibly adding to hip impingement and instability [70].

Figure 4.

A decrease in the femoral neck shaft angle will decrease the femoral height while increasing offset. This construct will increase the magnitude of the abductor lever arm [69].

Figure 5.

Notice how changes in neck shaft angle will change the femoral neck length.

Figure 5a shows 2 hip templates of the same implant with a change in neck shaft angle. Notice how the “offset” (125 degree) neck shaft angle will cause neck changes to primarily affect offset and little length is changed.

Figure 5b shows the “offset stem” in relation to the cup center of rotation. Notice the angle of the shaft is 125 degrees and changes in neck length primarily change offset with little change to length.

Figure 5c demonstrates a “standard offset” (131 degree) neck length. Notice how neck length changes will affect both length and offset more evenly.

4.3 Head size and instability

The value of larger femoral heads in THA has been increasingly recognized over the last 60 years. Since the 1960s, femoral head size increased from the original 22 mm to an average size of 32 mm by the mid 2000s [71]. Although some registry data reports the most common head diameter to be 32 mm, the use of 36 mm heads has been increasing. The AJRR recently reported a 36 mm head as the most commonly implanted size in the United States [72, 73]. Increasing femoral head size improves stability in THA through two main mechanisms. First, a larger diameter head is more deeply seated into the acetabular cup, requiring an increase in linear translation (also known as “jump distance”) in order for a dislocation to occur [74]. Second, an increase in the femoral head diameter, while maintaining a constant neck diameter, increases the head to neck ratio allowing for a wider impingement-free arc of motion [75].

The stabilizing effect of large femoral heads is well documented in the literature. A randomized controlled trial of 644 patients found that 36 mm heads resulted in a significantly decreased rate of dislocation when compared to 28 mm heads (1.3% versus 5.4%, p = 0.012) at 1 year follow-up [76]. Several recent registry studies have further emphasized the advantage of large femoral heads [36, 72, 77]. A study published from the Dutch Arthroplasty Registry reported significantly decreased dislocation risk with 32 mm heads when compared to those using 22–28 mm [36]. These authors noted further improved stability with a 36 mm head compared to 32 mm when evaluating operations performed through the posterolateral approach. Another recent investigation using the Danish Hip Arthroplasty Register also favored 36 mm heads compared to 32 mm, demonstrating a reduced dislocation risk within 2 years of primary THA [77].

Although these studies emphasize the stabilizing advantage of a large femoral head, the surgeon must weigh this advantage against the concern for increased wear characteristics and late failure [78, 79]. Several studies have determined 36 mm and larger heads to exhibit increased frictional torque and volumetric wear versus heads of 32 mm and smaller [80, 81]. The surgeon must also be aware that with a fixed acetabular component the space available for polyethylene decreases as the femoral head size increases. This will lead to a thinner polyethylene liner implanted if a 36 mm head is used (Figure 2A and B). A recently published study from the Australian Regstristy found that 36 mm heads had a statistically higher risk of late revision when examining metal on cross-linked polyethylene constructs versus ceramic head components [79]. Although more long-term data is needed, this study suggests that when using femoral heads of 36 mm or more, the surgeon may consider ceramic femoral heads for the best combination of stability and longevity.

Several strategies can be employed to achieve the stabilizing advantage of a large femoral head in total hip arthroplasty. Although some authors advise reaming up to achieve an acetabular component of sufficient size to accommodate a 36 mm head, there are concerns that this can change the center of rotation and biomechanics of the hip [82]. Odri et al. found that patients experienced significantly more postoperative pain, especially anterior iliiopsoas impingement, when the implanted cup was 6 mm or more larger than the native femoral head [83]. Authors have therefore advised implanting a cup that is no larger than 4 mm above the measured diameter of the femoral head [82]. In patients whose anatomy cannot accomodate a large acetabular shell, surgeons can employ several strategies to achieve an increased head size. This includes the use of thin polyethylene liners, metal on metal bearings, and dual-mobility implants. Despite concerns for increased liner wear and fracture, a recent report evaluating the use of large femoral heads with thin polyethylene liners at average 8.5 year follow up, noted a 100% survival rate when using liner failure as an endpoint [84]. Metal on metal bearings, which allow a head size closer to that of the native femoral head. These implants have displayed low rates of dislocation but their utilization has waned due to concerns of adverse reactions to metal debris (ARMD) [85, 86]. Dual mobility constructs allow for large femoral head diameter in addition to an increased arc of motion, and will be discussed more thoroughly later in this chapter.

4.4 Acetabular component

Postoperative hip stability depends on accurate placement of the acetabular component [87]. Factors that contribute to malpositioning may include intraoperative patient-positioning, abnormal pelvic anatomy and body habitus [88]. Surgeons continue to aim for an acetabular position of 40 degrees +/− 10 degrees of abduction and 15 degrees +/− 10 degrees of anteversion [89]. One technique includes positioning the patient in the center of the room with the sides of the operating table paralleling the walls of the OR. This allows the surgeon to base the version and inclination from the walls and the floor of the operating room [90]. To perform this technique, the patient must be positioned in a stable lateral decubitus position with the pelvis directly perpendicular to the floor. However, variations in pelvic position make balancing and stabilizing the patient in this position difficult. Additionally, inconsistencies in the size and shape of the operating room may alter the surgeon’s perception of patient orientation [90]. Therefore an alignment rod attached to the insertion handle of the cup has proven to be more accurate in comparison to free hand cup placement [91]. This rod allows the surgeon to more appropriately align the guide to visualize anteversion and inclination based on the floor and longitudinal axis of the patient. However, both techniques are sensitive to patient positioning and orientation of the pelvis.

Moreover, use of native pelvic anatomy increases accuracy in comparison to referencing external factors. One landmark often referenced to establish native pelvic anteversion is the transverse acetabular ligament (TAL) [92]. This landmark is independent of pelvic positioning and pelvic tilt. When using the TAL as a reference for anteversion and depth, Archbold et al. noted a 0.6% dislocation rate in 1000 consecutive patients [92, 93]. Other intrinsic pelvic landmarks such as the superior acetabulum, acetabular sulcus of the ilium and pubis have been reviewed but not widely adopted due to the large variability from osteophyte production [94]. Many are now starting to look towards intraoperative imaging and computer guidance to try to reduce surgeon error.

4.4.1 Acetabular offset

Acetabular offset, defined as the distance between the COR of the femoral head and the center of the pelvis, can be an important contributor to the stability and overall forces on a total hip arthroplasty. Charnley’s traditional techniques recommended medialization of the acetabular component in order to reduce the joint reactive force (JRF) on the hip [95]. However, this reduction in acetabular offset can result in increased impingement, reduced ROM, loss of soft tissue tension, and an increased risk of dislocation [96, 97, 98, 99].

An astute surgeon should attempt to re-establish the patient’s anatomic global offset (GO), the sum of femoral offset and acetabular offset. A decrease in GO after THA has been shown to result in loss of soft tissue tension and abductor function [100]. When medialization of the acetabulum is necessary, particularly in the setting of significant hip dysplasia, a stem with greater femoral offset is often required to restore the global offset and optimize stability of the hip joint [99]. Excessive medialization of the acetabular cup can increase the risk of impingement, a known risk factor for dislocation. In a computer model simulation, Kurtz et al. determined a decrease in acetabular offset to be the greatest risk factor for increased bony impingement [101]. Even restoration of global offset via increased femoral offset failed to fully restore range of motion before impingement. The study listed a deepened acetabular component leading to premature impingement of the femoral neck on either bone or soft tissue of the pelvis. With these ideas in mind, anatomic positioning of the cup with preservation of acetabular offset is advised.

4.5 Robotics/fluoroscopy and navigation cup position

With increased emphasis on proper positioning of components, there is growing interest in technology that allows more accurate and reproducible placement of total hip arthroplasty components. Free-hand cup positioning can be inaccurate and inconsistent, with one study finding that only 50% of the components were accurately placed in both anteversion and abduction target zones [23]. Techniques to improve component positioning can be separated into three categories: fluoroscopic guidance, computer navigation, and robotic-assisted total hip arthroplasty.

One advantage of the anterior approach is the ease of use of fluoroscopy during the operation. Rathod et al. found the use of fluoroscopy during an anterior approach significantly increased accuracy in cup placement compared to a non-guided posterior approach [102]. In addition, fluoroscopic guidance using the anterior approach has been shown to be more accurate than the use of fluoroscopy in the posterior approach due to the supine position allowing for a more accurate representation of a standing AP pelvis compared to the lateral position [103]. A recent study found the fluoroscopic assisted anterior approach to be as accurate at placing the cup into a target safe zone as a robotic-guided operation [104].

Computer navigation, used with or without the assistance of imaging, commonly relies on intra-operative anatomic landmarks and surgeon guided input reference points to aid in component positioning [99]. Several studies have reported computer navigation to result in more accurate acetabular component placement when compared to freehand methods [105, 106]. Robotic guidance combines computer navigation with the input of a robotic-assisted arm. Most contemporary designs are semi-active, where the robotic apparatus assists in certain actions while still requiring operation of the system by the surgeon [23, 104]. Similar to other technology guided techniques, robotic-assisted THA has shown increased accuracy of cup positioning versus manual techniques [107]. In addition to cup placement, these surgical technology enhancements can assist with achieving more accurate leg lengths, global offset, and combined anteversion measurements [108, 109].

Despite improved accuracy of component placement, the clinical advantages of technology assisted total hip arthroplasty continues to be debated. In a randomized controlled trial comparing 62 computer navigated THAs to 63 manual THAs, Lass et al. found no difference in dislocation rates at a minimum of 2 year follow up [110]. In their cohort of 2247 patients, Shaw et al. found robotic assisted THA to result in significantly lower rates of dislocation (0.6%) versus manual THA (2.5%, p < 0.05) [111]. Another recent study compared THAs performed through the posterior approach using robotic assisted, computer navigated, and manual techniques [112]. The authors found robotic-assisted posterior THA to have a statistically significant decrease in reoperation due to dislocation compared to the manual THA cohort (OR = 0.3,p < 0.05). Interestingly, this difference was not seen when comparing the computer navigated cohort to the manual group. The authors have proposed that the influence of robotic THA goes beyond improved cup positioning and warrants further study. These surgically assisted technology enhancements continues to increase in popularity, and continued high-level studies are needed to elucidate whether it provides sufficient advantages to outweigh the higher initial costs.

4.6 Acetabular liners: Added stability when needed

After satisfactorily implanting the acetabular component, the surgeon needs to make the decision on the type of liner used. Many manufacturers provide different acetabular liner options to allow the surgeon to recreate native anatomy and maximize hip stability. It is important that the surgeon become familiar with the liner options available when preparing for a case. Most implant manufactures allow for neutral, lipped, lateralized, oblique, constrained and dual mobility liners.

A neutral liner should be used when the surgeon is satisfied with the acetabular position, the hip stability, and in instances where the patient has no increased risk of dislocation. This liner allows the patient to have the greatest range of motion since the implant will sit flush with the acetabular component but does not give the surgeon any added stability [113]. Some surgeons prefer to use a “lipped or high walled liner” to allow for increased stability due to a larger jump distance in a discrete quadrant (Figure 6). With this liner option the surgeon can position the “elevated lip” to the area concern. This liner may increase intraoperative stability and allow the patient an additional 8 degrees of internal rotation if placed in the posterior quadrant [114]. From a posterior approach, the elevated liner should be placed in the “4 o’clock” position with a left hip and an “8 o’clock” position in the right hip [115]. It is important to note that this increased jump distance will also cause impingement in the area of increased elevation and may lead to dislocations.

Figure 6.

Left is a neutral liner. Right is a high walled liner.

In acetabular protrusio cases, native or iatrogenic from over reaming, or in cases where the patient has increased native offset, the surgeon may choose to use a lateralized liner. The lateralized liner has increased polyethelene in the medial portion of the implant and circumferential coverage. It is important to note that the lateralized liner will increase offset and length based on the acetabular implant position. In cases where the acetabular component is horizontal then the lateralized liner will increase overall hip length and in contrast in a vertical acetabular component orientation the lateralized liner will primarily increase offset. The lateralized liner should be considered when the abductor soft tissues are lax and the surgeon has already used a high offset implant [116]. It is important to note that a lateralized acetabular liner will increase the body weight moment arm and has been shown to increase joint reactive forces and thereby increase polyethelene wear rates [117].

In cases where the surgeon wishes to maintain a mildly malpositioned acetabular component an oblique liner may be used. When available, this option can become important in both primary and revision situations. The oblique acetabular insert has 180 degrees of coverage and reorients the range of motion in the direction of the obliquity [10]. Some surgeons have found this liner option to be quite useful in hip dysplasia since these patients may have highly irregular bone stock. This liner option allows the surgeon to place the acetabular component in a position that maximizes bony contact while reorienting the range of motion to a more functional “safe zone” [118].

Constrained liners are designed to physically lock the femoral head into the acetabular liner with the use of a metal ring [119]. These liners allow the surgeon the greatest amount of hip stability with the most amount of stress at the bone/implant interface. Surgeons should be aware that these liners are not indicated in situations with implant malposition or hip impingement. The primary indication for these constraint liners is neuromuscular disorders, abductor deficiency or intraoperative multidirectional instability without hip impingement or implant malposition [113]. This increased constraint is commonly used in more difficult hips and has been shown to have higher revision rates from several mechanisms of failure [120]. Locking ring failure from polyethelene wear or repetitive impingement and aseptic loosening causing cup migration or pull out are known failure mechanisms [88, 121, 122].

4.7 Dual mobility

Dual Mobility (DM) articulations were first designed by Bousquet in 1974. This design capitalizes on the principle of low friction arthroplasty, which favors a small femoral head, and the improved stability given with increasing the femoral head size [123]. The DM articulation achieves both increased stability and decreases wear by featuring two articulating surfaces within a fixed acetabular shell. These shells articulate with a large polyethylene ball; within the polyethylene ball sits another small (generally 22–28 mm) metallic femoral head. The benefits of DM include reduced rates of instability while maintaining longevity [124].

Modern DM articulations offer modular highly polished liners that are compatible within prior titanium acetabular shell designs. Prior generations—sometimes referred to as anatomic DM—featured monoblock cobalt chrome acetabular components. A unique challenge of these anatomic components was implantation due to difficulty with verification of component seating. The primary benefit of a monoblock component is a theoretical decrease in metal ions due to the disappearance of differing metal interfaces; however, short term series have not found significant differences to date [125, 126, 127].

Perhaps the most pertinent application for a dual mobility liner is in the setting of femoral neck fracture. Femoral neck fractures present within a patient cohort of generalized muscular insufficiency, recurrent falls, spasticity from immobility, neurologic disorder, and cognitive decline. Dislocation rates for total hip arthroplasty within femoral neck fractures have been published as high as 9% but can be lowered to 1.2% with the use of a dual mobility construct [124, 128, 129, 130, 131]. New literature has interestingly shown lower instability rates in DM total hips constructs over traditional hemiarthroplasty cohorts [129, 132]. It is therefore recommended to utilize DM constructs in total hips for femoral neck fracture [132, 133].

Fixed Spinopelvic alignment and its implications on hip stability has become an increasingly studied topic. In normal anatomy, the pelvis dynamically tilts posteriorly to increase acetabular coverage during a seated position to allow femoral clearance. In contrast while the patient is in the standing position the pelvis assumes a more neutral orientation [134, 135]. This native motion protects against femoral dislocation with hip flexion; however, patients with rigid spinopelvic alignment have repeatedly demonstrated to be at an increased risk of instability [136]. Dual mobility articulations have demonstrated significant improvement in hip stability within patients with fixed spinopelvic pathology [137].

Increasing data suggests that modern DM implants have longevity and appropriate wear characteristics; despite having two articulating surfaces which increases the risk of volumetric wear [138]. Limited retrieval studies demonstrate a wear rate similar to traditional cementless liners at 15 years [139]. Furthermore, systematic review suggests overall survivorship of 98% at mean followup of 8.5 years (2 to 16 years). The most common cause of revision was aseptic loosening at 1.3% followed by intraprosthetic dislocation at 1.1% [124]. Overall, DM total hip arthroplasties are a viable option with a proven track record of longevity and an ideal clinical application for patients at increased risk of instability.

5. Soft tissue procedures

To optimize hip stability, attention should be drawn to the soft tissue integrity and tensioning around the hip. Approaches that utilize the lateral decubitus position require meticulous capsular repair. Critics have stated capsular repairs will ultimately fail or lead to an unnecessary increase in surgical time; however, this repair has demonstrated less blood loss, decreased dislocations, and better functional outcome scores [140, 141]. In contrast, supine approaches have not shown this increased benefit [142]. Schwartz et al. published a randomized controlled trial regarding capsular repair vs. capsulectomy utilizing the direct anterior approach noting no difference in outcomes [143]. The increased stability from the direct anterior muscle sparing approach maybe from the preservation of the short external rotators or from the fluoroscopic guidance of intraoperative implant positioning. Ultimately, the data is unclear whether capsular retention and repair is necessary for post-operative hip stability using the direct anterior approach.

Even when the femoral and acetabular components are appropriately oriented, restoration of length and offset are needed to recreate the mechanical advantage of the abductors [144]. Abductor tensioning is affected by the sizing and positioning of both the femoral and acetabular components. Poor abductor repair, failure of trochanter osteotomies, and destruction of the greater trochanter from fracture or osteolysis will adversely affect this tensioning [145]. In severe cases of abductor deficiency, soft tissue transfers may be needed to increase strength and stability of the hip joint. A transfer of the anterior ½ of the gluteus maximus to the greater trochanter has been described to increase lateral stability and to assist with Trendelenburg gait [146]. It is also possible to perform transfers such as transferring the anterior half of the gluteus maximus to the greater trochanter to increase lateral stability and to assist with issues of Trendelenburg gait [147].

Although rarely required for a primary hip arthroplasty, a greater trochanteric osteotomy is indicated to remove well fixed implants for hip revisions. Robust fixation of this osteotomy is crucial to avoid trochanteric nonunion which can result in pain, hip weakness, and hip instability [148]. In cases of abductor weakness or trochanteric nonunion, an advancement may be considered. Dennis and Lynch describe a greater trochanter advancement surgery specifically in patients who have postoperative hip weakness and instability [146].

6. Postoperative hip instability

6.1 Hip precautions and does anterior hip need precautions

Hip precautions have often been utilized to help aid in prevention of dislocation in the acute postoperative period. This is often done in patients who undergo a posterior approach where the short external rotators and the posterior capsule is compromised. Many physicians use these precautions. A recent prospective randomized trial from Journal of Arthroplasty 2022 examined 346 consecutive patients all via posterior approach to the hip with a mean follow up of 2.3 years. This study demonstrated that if intraoperative hip stability was obtained at 90 degrees of flexion 45 degrees internal rotation and 0 degrees of abduction, postoperative hip precautions are no longer necessary. This study, however powerful, excluded patients with previous lumbar fusion, scoliosis or abductor insufficiency [149]. Mounts 2022 study is in accordance with another recently published a systematic review that included 6900 patients. This study demonstrated no statistically significant decrease in dislocations with the use of posterior hip precautions [150].

Since anteriorly based approachs are often regarded as a more stable approach post operatively, surgeons have questioned the need for precautions post operatively. Talbot et al. studied 499 cases of primary total hip arthroplasty done through an anterolateral approach and documented the dislocation rate when restrictions were not imposed. There were 3 early dislocations (within 6 weeks of surgery) all of which were close reduced, and every patient subsequently achieved a stable hip without further intervention [151]. Restrepo et al., also demonstrated a 0.16% dislocation rate which is significantly lower than the 2% overall that was found to occur within the 1st year by Maratt et al. in anterior and posterior approaches [34, 152]. The evidence for hip precautions after an anterior-based hip approach seems to be in favor of not requiring restrictions.

6.2 Recognition of postop instability from infection, poly wear, ALTR

To conclude, the surgeon must correctly identify the etiology of the instability to direct treatment. Early postoperative instability is likely due to component malpositioning or acute infection [116]. In cases of late stage instability the surgeon should consider component subsidence, aseptic loosening, osteolysis, indolent infection or the development of an adverse local tissue reaction (ALTR). Acute infection may be challenging to diagnose if obvious wound complications are not present [153, 154]. Both acute and chronic infection can present with loosening of one or both the acetabular and femoral components which may require staged revision. Another important cause of instability is aseptic loosening from polyethylene debris leading to macrophage induced osteolysis. This can ultimately lead to movement or dislodging of the implants which should be closely evaluated and may require revision surgery. Osteolysis can destroy available bone stock requiring the surgeon to become facile with bone grafting, cages, or even custom triflange implants for the acetabulum [155]. In the case of femoral bone loss there may be a need for diaphyseal engaging implants, bone grafting or even proximal femoral replacement [156]. Another potential cause of instability is the development of an ALTR from metal on metal (MOM) bearing surfaces. Diagnosis of ALTR is made from clinical history, radiography and serum metal ion levels. If surgery is deemed necessary, additional information may be obtained from ultrasound and metal artifact reduction sequence magnetic resonance imaging (MARS-MRI) to evaluate soft tissue destruction and possible need for augments or constraint due to abductor deficiency. Ultimately if serum ion levels continue to rise or patient functionality declines the patient will require revision surgery [157]. Although post-operative hip instability frequently requires revision surgery it is important to identify the root cause. This will allow the surgeon better surgical preparation, more readily available implants and the ability to manage infection with possible staged surgery or prolonged IV antibiotics.