The number of total hip arthroplasty (THA) procedures, a commonly performed elective orthopaedic procedure, continues to rise^1-4). Although excellent outcomes in terms of implant survivorship and patient reported outcomes have been achieved using primary THA, there is a risk that revision THA (rTHA) will be required. Studies conducted in the USA have reported dislocation rates ranging from 0.2% to 10%⁵⁾. Hip instability, which remains a leading cause of rTHA, is expected to increase in the coming years^2,6,7).

According to traditional teaching, in order to reduce impingement and therefore risk of dislocation, the cup should be placed within a “safe zone” as described by Lewinnek et al.⁸⁾ with a target cup position of 40°±10° inclination and 15°±10° anteversion. However, this has recently been questioned as high rates of dislocation within this “safe zone” have been reported in the literature, indicating the potential involvement of other factors⁹⁾. Most atraumatic dislocations are likely the result of atypical pelvic kinematics that cause changes in functional cup positioning^10,11). Because patient-specific cup positioning has been associated with decreased risk of impingement and instability leading to dislocation and rTHA, as an effort to reduce the burden of instability after THA, the hip-spine relationship is a significant consideration during preoperative planning¹²⁾.

Two specific aspects of the spine that are vital for improving the characteristics of impingement and the stability of the implant should be considered when assessing the position of the pelvis^13-16). The first consideration is spinal rigidity, characterized by a change in sacral slope (SS) of less than 10° when shifting from standing to sitting^17,18). This metric can be helpful in assessing the movement of the pelvis during its transitions between these positions. When the spine is flexible, the pelvis rolls posteriorly with movement from a standing to a seated position, which increases anteversion, allowing the hips to flex until the femurs are parallel with the ground¹⁶⁾. Conversely, in a rigid lumbar spine, pelvic movement is reduced, which can lead to impingement of the anterior proximal femur on the anterior rim of the cup¹⁹⁾. One study reported that the risk of implant impingement and subsequent dislocation is increased for patients with a previous lumbar spinal fusion (LSF) or lumbar degenerative disc disease^20-24). A dislocation rate of 8.3% has been reported in THA patients with previous LSF with a mean follow-up period of five years²⁵⁾. In a study examining the outcomes of 207,285 THAs, a history of spinal fusion was the most significant independent risk factor for dislocation within the first six months following THA²⁶⁾. These findings place emphasis on the importance of considering the history of spinal interventions prior to THA.

The second issue to consider is the degree of spinal deformity or sagittal spinal imbalance. Spinal deformity can be identified by a difference of greater than 10° between pelvic incidence (PI) and lumbar lordosis (LL), or an anterior pelvic plane (APP) without neutral or vertical alignment²⁷⁾. As PI is a morphological parameter that does not change with patient age, position, or deformity, a loss of LL is commonly the driver of sagittal spinal imbalance²⁸⁾. Degenerative changes that occur in the spine can cause a gradual decrease of LL leading to application of compensatory mechanisms in posture for maintenance of a balanced position with the body centered over the pelvis²⁸⁾. Increased posterior tilt of the pelvis when standing, resulting in a change in functional cup position, is an example of a compensatory mechanism²⁹⁾. An associated increased risk for dislocation has been described in the literature with regard to this high-risk patient population^30-32).

When examining spinopelvic kinematics for THA, it is important to consider the potential for postoperative changes in spinopelvic mobility, which may often be attributed to the preoperative presence of a hip flexion contracture. This condition, which can limit pelvic mobility, may be resolved during surgery, thereby altering the dynamics of pelvic motion in both the hip and pelvis. Of particular interest, the release of hip flexion contractures can lead to an increase in spinopelvic mobility because these contractures typically limit posterior pelvic rotation during ambulation. Sariali et al.³³⁾, who observed significant post-THA alterations, reported an average change in SS of 5° when standing and 3° when sitting. Similar studies have demonstrated that pelvic tilt measurements can vary by up to 20° postoperatively^15,34). Consequently, in the evaluation of the hip-spine relationship, identification and consideration for the presence of a hip flexion contracture is critical, as this may resolve following THA, altering the pelvic motion.

Preoperative assessment of the spinopelvic kinematics is essential to reducing the risk of instability in management of patients undergoing THA. The Hip-Spine Classification for THA was developed as an effort to provide a simple framework for the preoperative evaluation of the spinopelvic relationship and a guide for intraoperative management of patients undergoing THA³⁵⁾. The results of this prospective, multicenter study, validated utilizing a cohort of 2,081 patients, showed low rates of dislocation even for high-risk patients. This classification system can be helpful in the effort to optimize the orientation of components, selection of bearings, and potentially reduce the occurrence of postoperative hip instability. When using this classification system, patients are grouped according to four distinct categories, determined by two key factors: spinal stiffness, categorized as either A or B, and spinal deformity, classified as 1 or 2. This categorization is systematically shown in Table 1, following the framework of the Hip-Spine Classification.

Table 1 . The Hip-Spine Classification System Developed by Vigdorchik et al.³⁵⁾.

Group	Classification	Pathology	Inclination, anteversion target (°)	Special considerations
1A	Normal spinal alignment and mobility (PI-LL <10°; ΔSS >10°)	Normal anatomy and mobility	40°, 20°-25°	None, standard THA component positioning
1B	Normal spinal alignment and stiff spine (PI-LL <10°; ΔSS <10°)	“Stuck standing”: Stiff spine needs more inclination and anteversion.	45°, 25°-30°	Utilization of a 40 mm or larger femoral head (depending on cup size and poly thickness) and restoration or the consideration of slight (<5 mm) increase in native offset are recommended.
2A	Flatback deformity and normal spinal mobility (PI-LL >10°; ΔSS >10°)	Anterior pelvic tilt: From hip flexion contracture, will resolve postoperatively.	40°, 20°-25°	Native anatomy will correct itself over time following the THA.
		Posterior pelvic tilt: Spinal deformity will cause more functional cup anteversion.	40°, 20° unless anterior APPt >13°, then target less than native anatomy
2B	Flatback deformity and stiff spine (PI-LL >10°; ΔSS <10°)	“Stuck standing”: Spinal deformity and stiff spine will cause more functional cup anteversion.	For posterior APPt of <13°: 40°, 20°-25°	Largest head size possible (>40 mm) or dual mobility components and ensuring that the offset is not decreased during surgery.
			For posterior APPt >13°: Less than native anteversion and inclination should be targeted. 45° of inclination and 25°-30° of anteversion relative to the functional pelvic plane.

PI: pelvic incidence, LL: lumbar lordosis, SS: sacral slope, THA: total hip arthroplasty, APPt: anterior pelvic plane tilt..

Identification of patients with hip-spine pathology is important in the comprehensive preoperative assessment of candidates for THA. Therefore, the following imaging protocol is recommended for all patients undergoing THA.

• Supine anteroposterior (AP) pelvis

• Standing AP pelvis

• Standing lateral radiographs

• Relaxed-sitting lateral radiographs

However, when X-rays are ordered selectively, priority should be given to: individuals with prior spine surgery, known or suspected spinal disease, prior rTHA indicated for instability, females >75 years, and patients with an “outlet” view on a supine AP pelvic radiograph suggestive of a posterior pelvic tilt^13,36). It is important to note that spinal fusion does not always indicate rigidity in all patients with prior spinal fusion; conversely, a rigid spine does not always correlate with antecedent spinal fusion³⁷⁾. Last, it is important to note that 59% of patients undergoing THA have normally aligned lumbar spines and 18% of patients have three or more risk factors for adverse spinopelvic mobility^32,38).

1. Step 1: Detect the Presence of Spinal Deformity

Supine AP pelvis, standing AP pelvis, and standing lateral radiographs (Fig. 1) should be obtained for evaluation of pelvic tilt, rotation, or obliquity and anterior or posterior pelvic tilt. Natural tilt of the pelvis may show significant variation in the supine and standing position, even without gross spinal pathology. Standing AP and lateral pelvis views better represent the functional position of the pelvis during activity (Fig. 1). Previous surgery or lumbar degenerative disease can be identified by including the lumbar spine in the AP radiograph of the pelvis.

Figure 1. Standing anteroposterior pelvis and standing lateral radiographs for the evaluation of pelvic tilt, rotation, and obliquity.

The absence or presence of spinal deformity can be confirmed by drawing the APP, defined by the midpoint between the two anterior superior iliac spines and the pubic tubercle (Fig. 2). APPt (or pelvic tilt) should be defined as neutral, tilted posterior, or tilted anterior. Specifically, in patients with no deformity, the APP will be vertical (neutral pelvic tilt) or parallel to the coronal (functional) plane of the body. AP and lateral X-rays must include up to at least L1 proximally, and down to the pubic symphysis distally, and can be normal flat plate radiographs or stereoradiographs (EOS Imaging). A standard 36-inch cassette is sufficient if stereoradiography is not available³⁹⁾.

Figure 2. Standing (A) and sitting (B) lateral measurements for evaluation of the hip-spine relationship. APPt: anterior pelvic plane tilt, SS: sacral slope, PI: pelvic incidence, LL: lumbar lordosis.

Identifying a PI-LL mismatch is critical when examining for spinal deformity. PI is considered a constant parameter that enables estimation of a patient’s physiologic range of spinopelvic motion in the sagittal plane. PI is defined as the angle between a line drawn from a point halfway between the femoral heads to the middle of the S1 endplate and another line perpendicular to the superior S1 endplate. PI is also related to SS and PT by the following formula: PI=PT+SS. LL is defined as the angle between a line drawn at the superior L1 endplate and another line drawn at the superior S1 endplate. A PI-LL mismatch of >10°, measured on the standing lateral spinopelvic image indicates the presence of a “flatback spinal deformity” (Fig. 3).

Figure 3. Sitting and standing radiographs in a patient with flatback deformity. This figure illustrates the key spinal parameters: anterior pelvic plane tilt (APPt), sacral slope (SS), lumbar lordosis (LL), and pelvic incidence (PI). A discrepancy greater than 10° between PI and LL is indicative of flatback deformity. The radiographs displayed show a PI-LL mismatch of 14°, confirming the presence of the deformity in the patient under study.

2. Step 2: Detect the Presence of Spinal Stiffness

Spinopelvic mobility is defined by changes in SS, between standing and sitting lateral radiographs. Change of SS is analogous to changes in APPt and spinopelvic tilt. SS represents the angle between a line parallel to the ground and one parallel to the superior S1 endplate. SS decreases with posterior rotation of the pelvis. “Spinal stiffness” is defined as a change in SS of <10° between the standing and sitting positions. Spinopelvic tilt is defined as the angle on a lateral radiograph between the vertical axis and a line between the center of the S1 endplate and the point between the center of the femoral heads. It should be noted that stiffness of the spine may also affect patients with no history of instrumented fusion secondary to degenerative spine disease and that patients with short segment instrumented fusions can still maintain spinopelvic mobility.

While patients categorized as 1A have normal spinal alignment, normal pelvic mobility and not considered a higher risk for postoperative dislocation, those belonging to groups 1B, 2A, and 2B are considered high risk due to their spinal stiffness and/or deformity. The Hip-Spine Classification and the appropriate cup position targets to aim for in each specific group are outlined in Table 1.

1. 1A: Normal Spinal Alignment and Mobility

For patients with neither spinal stiffness nor pelvic or spinal deformity, determined by a PI-LL of <10°, neutral APPt and a ∆SS of >10°, a traditional hip replacement can be performed with no need for component modifications (Fig. 4). An acetabular component target of 40° inclination and 20°-25° anteversion is recommended in such cases.

Figure 4. Standing (A) and sitting (B) lateral EOS stereoradiographs imaging of a patient in the 1A group. Spinal deformity is not present with a PI-LL <10° (PI-LL=5°) and a relatively neutral pelvic tilt of –3°. Spinal mobility between standing and seated positions is appropriate with a ΔSS >10° (ΔSS=21°). APPt: anterior pelvic plane tilt, SS: sacral slope, LL: lumbar lordosis, PI: pelvic incidence.

2. 1B: Normal Spinal Alignment and Stiff Spine

For patients with a normal spinal alignment (PI-LL <10°) and spinopelvic stiffness (∆SS <10°) caution should be taken to avoid impingement and minimize the risk of dislocation. Patients in Group 1B had limited rollback of the pelvis when assuming the seated position and the pelvis was oriented in a fixed anterior tilt position in the sagittal plane resulting in a more horizontal position of the acetabulum¹¹⁾. Therefore, positioning the acetabular component with a traditional target of 40° of inclination and 20°-25° anteversion can result in impingement of the flexing proximal femur on the anterior acetabular rim when sitting. In this group the target should be increased anteversion of 25°-30° and a slight increase in inclination of 45° to reduce the risk of posterior dislocation^16,29,39,40). Caution should be taken to avoid an excessive increase in anteversion, which may result in iatrogenic posterior impingement and anterior instability. Utilization of a 36 mm or larger femoral head (depending on cup size and poly thickness) and restoration or the consideration of a slight (<5 mm) increase in native offset are also recommended^35,41,42).

3. 2A: Flatback Deformity and Normal Spinal Mobility

Patients with spinal deformity (PI-LL >10°) and normal spinal mobility (∆SS >10°) can have either increased anterior pelvic tilt, secondary to a flexion contracture of the hip or an increased posterior pelvic tilt, typically due to compensation for a flatback deformity. From the standpoint of surgical planning and execution targeting native anatomy with an inclination of 40° and anteversion of 20°-25°, reflecting the presumed postoperative resolution of deformity is considered appropriate. However, if the posterior pelvic tilt is greater than 13°, less anteversion and inclination compared with native anatomy is required to prevent an increased risk of anterior dislocation due to the component being more functionally open. A posterior pelvic tilt of >13° is large enough to significantly alter the trajectory of a component positioned at 40° inclination and 20° anteversion, resulting in its deviation from its established safe zone^16,29,43,44).

4. 2B: Flatback Deformity and Stiff Spine

Patients in the 2B group (Fig. 5) presented with spinal deformity (PI-LL >10°) and a stiff spine (∆SS <10°) and represented the highest anatomic risk cohort for dislocation. Careful evaluation of posterior pelvic tilt in these patients is important for determining the functional position of the pelvis, which opens the acetabulum and increases inclination and anteversion. Therefore, a distinction must be made within the 2B group between patients with posterior tilt less than or greater than 13°. For patients with a moderate posterior pelvic tilt (APP <13°), a slight increase in anteversion is warranted to compensate for the stiff spine; however, this must be weighed against the risk of anterior instability when standing. The target for this cohort of patients is placement of the acetabular component at 40° inclination and 25°-30° anteversion relative to the functional pelvic plane. For patients with a large posterior pelvic tilt (APP >13°), the increase in functional inclination and anteversion is sufficient to remove it from the safe zone range. Therefore, the ideal target of cup anteversion and inclination should consider the degree of posterior pelvic tilt, with addition of less anteversion relative to the native anatomy with increasing posterior pelvic tilt. This population of patients should receive a target cup placement of 40° of inclination and 20°-25° of anteversion relative to the functional pelvic plane. Other factors to consider in this population include utilizing the largest head size possible or dual mobility omponents and ensuring that a decrease in offset does not occur during surgery.

Figure 5. Standing (A) and sitting (B) lateral EOS stereoradiographs imaging of a patient in the 2B group. Spinal deformity is present with a PI-LL >10° (PI-LL=14°) and a posterior pelvic tilt of –11°. Spinopelvic mobility is decreased with a ΔSS <10° (ΔSS=5°) from standing to seated. APPt: anterior pelvic plane tilt, SS: sacral slope, LL: lumbar lordosis, PI: pelvic incidence.

Mispositioning of the acetabular component and its adverse effects on stability is regarded as a significant contributor to dislocation following THA⁴⁵⁾. Conventional techniques and anatomic landmarks including transverse acetabular ligament, or technology assistance, can be used to achieve intraoperative execution of cup placement. In a randomized controlled trial, Meermans et al.⁴⁶⁾ examined the precision of acetabular component positioning using a freehand technique versus referencing the transverse acetabular ligament. The freehand method was reported to show greater variability, with a broader range of cup placement, with a mean anteversion angle of 21° (range, 2°-35°), as opposed to a mean of 17° (range, 5°-25°) when using the transverse acetabular ligament as a reference point, a statistically significant difference (P=0.004)⁴⁶⁾. While such referencing techniques may be useful for recreating native acetabular anatomy, this can become problematic in patients with spinal deformity and stiffness. As an example, in patients with significant posterior tilt and excessive native anteversion, attempting to recreate this native positioning may result in an excessively anteverted functional position of the cup. For those with abnormal spinopelvic kinematics, the safe zone may be a smaller target and patient-specific functional cup positioning may be required; computer-based navigation and robotics could be particularly useful in such cases^47,48).

Development of innovative digital solutions has led to advancements in the preoperative assessment of THA, particularly regarding the hip-spine relationship. These tools can leverage patient-specific anatomical and spinopelvic mobility data, for optimizing component placement tailored to the unique anatomy of the patient. Among these, the Cuptimize (Depuy Synthes) Hip-Spine utilizes four x-ray images (lateral standing, lateral sitting, AP pelvis supine, and AP pelvis standing) (Fig. 6) for stratifying patients according to the risk of dislocation⁴⁹⁾. Using four X-ray images (lateral standing, lateral sitting, AP pelvis supine, and AP pelvis standing) (Fig. 6) the software can be utilized to stratify patients based on risk of dislocation, which can be helpful to surgeons for more effective planning of cup positioning. The software can predict values for standing and seated position anteversion and inclination, which can be helpful for adjusting cup positioning accordingly during preoperative planning (Fig. 7). Integration with fluoroscopy can ensure accurate intraoperative placement, correlating with preoperative supine radiographs of the pelvis (Fig. 8).

Figure 6. Lateral seated (A) and standing (B) X-rays.

Figure 7. Cuptimize Hip-Spine Analysis preoperative planning screen. In this example, with a target cup position of 40° of inclination and 23° of anteversion, the predicted values in standing and seated positions is 41° of inclination and 25° of anteversion.

Figure 8. Intraoperative fluoroscopic image demonstrating acetabular cup positioning with seated and standing prediction of anteversion and inclination.

In addition to Cuptimize, several other tools also offer diverse approaches to assessing patient-specific spinopelvic mobility. ONE Planner Hip by Zimmer Biomet⁵⁰⁾, Intellijoint VIEW by Intellijoint Surgical⁵¹⁾, OPS (Optimized Positioning System) by Corin⁵²⁾, Mako Total Hip by Stryker⁵³⁾, and RI.HIP Navigation by Smith & Nephew⁵⁴⁾ each offer unique features and methodologies. Integration of these tools with their respective computer-navigated or robotic-assisted THA systems is being increasingly implemented in the effort to enhance the accuracy and effectiveness of surgical procedures^47,55,56). Selection of a specific tool can be dependent on various factors, including the surgeon’s preference, the specifics of the case, and the available technology.

Effective preoperative planning, a critical factor of THA, can enable the identification of unique anatomical challenges as well as precise determination of the size and placement of components. This will enable attainment of a better understanding of the specific needs of each patient, so that a surgical approach can be tailored accordingly.

No funding to declare.

No potential conflict of interest relevant to this article was reported.