Positioning of the acetabular component: Preoperative planning
By design, total hip arthroplasty (THA) requires the careful consideration of each patient’s unique anatomy. Surgeons must carefully plan their approach, customizing the procedure based on preoperative images, physical examinations, a patient’s history, and their own experience. Accurately templating the placement of the acetabular cup is critical to a successful THA. Anticipating potential complications and having the right instrumentation and device options on hand is also integral to good planning. We look at preoperative planning considerations that prioritize a patient’s anatomy and share expert opinion on the relationship between stiff/fused spines and cup placement.
For any surgical discipline, preoperative planning is critical to the success of a procedure. The AO Principles of Fracture Management remind us that planning is a focused endeavor that allows surgeons to contemplate their approach and “mentally rehearse the operation: Problems can be anticipated and avoided and alternative plans can be developed in case of arising difficulties” .
Aside from facilitating the delivery of a considered, patient-specific total hip arthroplasty (THA), the records generated by comprehensive planning can communicate the professionalism, thought, and motivation behind a surgeon’s approach, which could help in the defense against litigation .
Inaccurate cup placement is a persistent problem that has been linked to dislocations and bearing surface wear.  Jolles et al. reported dislocation risk to be 6.9 times higher if total anteversion did not fall between 40°–60° and stated that “ [s]urgeons should pay attention to total anteversion (cup and stem) of THA . Part I of this article series looked at the elements that influence accurate acetabular cup placement.
Callanan et al. found that out of 1823 hips, only 50 percent of the acetabular cups were placed within the safe zone (abduction (30°–45°) and version (5°–25°)) for both anteversion and inclination. In their study, low volume surgeons (two times higher risk), a minimally invasive approach (six times higher risk), and obesity (1.3 times higher risk) were contributing factors to malpositioned cups .
THA is a surgically demanding procedure and preoperative planning for acetabular cup placement involves consideration of several related factors. Bassam Masri, MD, and Head of Orthopaedics at the University of British Columbia, Canada, points out that,
“The most important technical aspect in planning hip replacement is restoring the biomechanics of the hip by restoring an anatomic center of rotation, restoring femoral offset, and equalizing leg lengths.”
That being said, let’s see how a surgeon’s preoperative plan can improve the likelihood that cup placement will be as precise as possible for each patient.
Preoperative, comprehensive patient assessment is vitally important, but this is tempered with the knowledge that too many tests risk fatiguing and burdening the patient, stressing clinical resources, and demand time to document and interpret. When assessing function, surgeons usually combine elements from a patient-reported questionnaire and functional tests .
New technologies, such as accelerometers, are making it possible to gather real-time information about a patient’s movement as they go about their lives . However, these types of technology are in early stages of adoption by the orthopedic field and despite being identified as feasible, they currently have little proof of efficacy, particularly in the post-THA rehabilitation phase .
Thorough medical history
Knowing a patient’s complete medical history provides insight into local and systemic risk factors and gives indication of potential issues that may arise during the procedure or that could complicate recovery . Comorbidities, such as diabetes or dementia , will influence surgical decision-making and should be discussed.
For example, a high body mass index (BMI)  and super obesity , preoperative cognitive impairment , functional dependency , gender and age , and even the type of health insurance carried by a patient  have been shown to impact outcomes and increase the risk of THA complications. A high pre-operative American Society of Anesthesiologists (ASA) score has also been correlated with a 10 times higher risk of dislocation .
A physical assessment should be performed to establish a patient’s baseline functionality. Solid understanding and documentation of preoperative parameters informs both the planning of the procedure, intraoperative decision-making, and determination of postoperative success.
There are myriad tests, evaluations, and questions that provide a surgeon with useful information when establishing baselines (see Table 1). Researchers have highlighted the fact that because there are so many protocols and corresponding variables for determining function, there is a need for an agreed-upon, standardized set of measures going forward .
Table 1. A selection of tests to consider for pre- and postoperative THA patient assessment. Complied from information (unless otherwise noted) in Table 1 and 2 of: Wylde V, Blom AW, Bolink S, et al. Assessing function in patients undergoing joint replacement: a study protocol for a cohort study. BMC Musculoskelet Disord. 2012 Nov:13;220. Used with permission under CC Attribution License 4.0.
Don’t forget to check for…
Targeted questioning and physical exams, in combination with imaging, should be performed to determine if the preoperative patient has dysplasia, limb length discrepancy (LLD) (see previous edition of Surgical Insights), and/or evidence of spinal disease. Congenital hip disease influences placement of the acetabular cup; THA in these patients requires “acetabular reconstruction [that] is technically demanding because of anterolateral bone deficiency and increased socket anteversion” .
Special consideration of the spine
The complex hip-spine relationship should be taken into consideration during preoperative planning. The lumbar spine range of motion and the relationship between the pelvis and the hip(s) should play a role in decisions around acetabular cup positioning. Mohamad Allami, MD, and Senior Consultant Orthopaedic Surgeon at the Alarabi Hospital for Surgical Specialty, Baghdad, Iraq, explains:
“In normal human beings the relationship between the spine and hip joint is a complex phenomena of kinematics by the articulation of the two through the sacroiliac joint. In principle, the pelvis moves backward with the spine during sitting from a standing position, which increases the distance between the socket and the femur during sitting, allowing a greater range of motion of the hip joint. Therefore, the restriction of this movement (e.g. severe spinal arthritis or previous lumbar spinal fusion), increases the risk of instability (dislocation or subluxation) and revision, and may negatively impact patient-reported outcomes.”
Impingement risks are higher in patients with spinopelvic imbalance . Patients with postoperative supine coronal cup inclination of ≥ 50° and anteversion of ≥ 25°, and those with a hypermobile pelvis, are more likely to experience this complication . Preoperative imbalanced sagittal alignment (larger sagittal vertical axis, larger pelvic incidence without lumbar lordosis, pelvic retroversion) has also been linked to poorer clinical outcomes such as dislocation and edge-loading wear [18, 19].
Stiffness or hypermobility of the spine are the root of spinal imbalance. Rivière et al. suggest correcting a severe imbalance with spinal surgery, if required, before the THA . Surgeons should take extra care to establish the correct intraoperative acetabular component inclination and anteversion, guided by each patient’s preoperative spinal mobility indicators, as this has been shown to effectively compensate for most spinal imbalance. It is worth noting that Stefl et al. reported that the acetabular component positions surgeons should “target for hypermobile hips are lower than normal…and for stiff hips are higher than normal” .
There is a subset of individuals who present abnormal pelvic motion but have no noticeable lumbar deformity. Gia et al. remind us that “normal lumbar kinetics [do] not necessarily suggest normal pelvic motion” .
Thoughts on THA in patients with stiff/fused spines
Research is showing that in patients with concomitant lumbar and hip pathology, where both lumbar spinal fusion (LSF) and THA are indicated, the sequence of these procedures significantly impacts the outcome of the THA [22, 23, 24]. If THA is performed before the LFS, dislocation and revision rates are lower compared to THA’s in patients with previous LSF. It is hypothesized that lumbar fusion may eliminate the protective flexibility of the lumbo-pelvic unit and cause impingement . In a case of spinal long fusion with pelvic fixation in a patient with previous THA, repeated dislocations were thought to be the result of a change in acetabular cup alignment after fixation, decreased pelvic tilt while sitting, or more forward tilt of the trunk when moving into standing from sitting . Take home message? If a patient needs both a THA and LSF, do the hip first.
Operative patient positioning is of great importance in cases of pre-existing spinal fusion. The position of the spine in relation to the hip joint, and limb length, impacts the acetabulum’s orientation (the inclination and anteversion angles); the socket’s position can be significantly altered in these patients.
Special care should be taken to anatomically position the acetabular component. Consider using a large head implant or Dual Mobility Cup system and perform repeated intra-optative trials to select the best fit to achieve a safe arc of moment of the hip joint and limb length equality before committing to the final implants.
In general, I prefer to use a dual mobility cup in patients with fused/stiff spines. This allows maximal hip mobility before impingement, and this reduces the risk of dislocation.
With a fused or stiff spine, spinopelvic motion is compromised, and the exact position of the cup that would yield maximal stability becomes unknown. For this reason, a dual mobility cup becomes crucial in minimizing the risk of dislocation.
The role of imaging
Preoperative imaging of a patient provides essential information on what to expect intraoperatively, in particular, variability in anatomy – no two patients are the same. Imaging assists in the formulation of a patient-specific strategy and allows a surgeon to identify appropriate materials and instrumentation they would like ordered and on hand.
When obtaining radiographic images (x-rays), generally anterior-posterior (AP) pelvic, cross-table lateral, and Dunn-views are most commonly obtained.  Mohamad Allami, MD, suggests,
“Anterior-posterior (AP) and lateral views of the planned hip joint, as well as the contralateral hip, to be able to appropriately determine leg length, acetabular and femoral component size and position, identify potential complications/deformities and additional implants that might be required during surgery.”
Proper patient positioning during the x-rays is very important, as is obtaining weight-bearing x-rays . Figure 1 illustrates how the leg’s degree of rotation influences the quality and usefulness of the image obtained.
Figure 1. Hip x-ray taken with leg in: (A) External rotation; (B) Internal rotation; and (C) Extra internal rotation over 15-20°. (B) is considered optimum image. Shaikh A. Preoperative Planning of Total Hip Arthroplasty. 2018. 10.5772/intechopen.76368. https://www.researchgate.net/publication/328797039_Preoperative_Planning_of_Total_Hip_Arthroplasty. Used with permission under CC by 3.0 License.
Three-dimensional (3D) imaging of complex anatomical areas, such as the acetabulum, can also be useful in planning; traditional radiographs do not fully capture osseous destruction or soft tissue structures . Computed tomography (CT) and/or magnetic resonance imaging (MRI) may alert a surgeon to unexpected anatomical anomalies not visible in an x-ray. Figure 2 provides a side-by-side example of each type of imaging.
CT and MRI imaging will generate 3D models, assisting surgeons in visualizing and understanding their patient’s acetabular anatomy; a useful tool particularly in cases of complex acetabular deformity . Some researchers even recommend constructing a 3D plaster model to better visualize pelvic destruction, especially for revisions in cases with complex acetabular defects . However, CT and MRI imaging are more costly options and not as widely available as x-rays throughout the world. Mohamad Allami, MD, finds that,
“A high-quality CT scan of the hemipelvis and the femur allows you to create a 3D blueprint of the hip. This is useful for cases of PSI as it helps you determine the optimal preparation of the acetabular and the femoral sites, for example, the femoral cut height and angle, femoral head anteversion, and acetabular site preparation in terms of depth, size, inclination, and anteversion.”
Figure 2. The same human pelvis, front imaged by: x-ray (left); magnetic resonance imaging (MRI) (center); and 3-dimensional computed tomography (CT) (right). Bjoertvedt- Own work. Available at: https://commons.wikimedia.org/w/index.php?curid=27198216. Used with permission under CC BY-SA 3.0.
A special note about acetabular retroversion (AR), which is the malorientation of the acetabulum in the sagittal plane and a sub-type of focal pincer deformity [31, 32]. Identifying this condition prior to THA is advised as it greatly influences outcomes and will impact a surgeon’s decision-making. The presence of AR should trigger mitigative considerations related to femoroacetabular impingement. To make an AR diagnosis, Direito-Santos et al. suggest evaluating X-rays for “the cross-over, the posterior wall, and ischial spine signs” and combining this with CT to identify the presence of lesions .
Templating: defining the puzzle pieces
The templating process is facilitated by, and indeed requires, “properly exposed, high quality standardized AP x-rays with set magnification” . Accurate templating is critical for several reasons [33, 34]. Firstly, it helps the surgeon predict appropriately sized implant components, generally within a range of plus/minus one size for the cup . Secondly, the angular positioning and insertion depth and height of the cup can be estimated, which is guided by the goal of restoring the patient’s native biomechanics (ie, center of rotation) .
Bassam Masri, MD, explains how accurate templating is critically related to correct component selection and positioning.
“Correct component selection is needed to achieve stable fixation and optimally position the acetabular component to restore the anatomic center of rotation. This will maximize joint stability and minimize wear. It is also important to select an implant with the correct offset, and to put it in the femur at the correct level by precisely templating and executing the femoral neck cut to restore offset and equalize leg lengths.”
THA templating was traditionally performed by superimposing acetate templates over an x-ray. This analogue planning can be effective ; however, careful attention must be given to magnification. If the images’ magnifications are not calibrated correctly, errors will occur .
Digital templating has been shown to be more accurate than analogue templating , some calling it the “current standard” . There are two digital approaches to templating generally employed by surgeons. Personal preference and availability and cost of the technology play a role in which is used.
Digital templating requires specialized software that electronically calibrates an uploaded radiograph to its own library of templates, digitally overlaying these two images. But, if external calibration markers are not correctly positioned, digital templating software will not eliminate magnification calibration errors , resulting in incorrect selection of component sizes . Stigler et al. recommend adding an anterior-posterior view during digital planning as this was found to “increase the correlation between preoperative planning and eventual implant sizes” .
When compared to analogue templating, researchers have reported that the digital method produces significantly superior planning of the vertical and horizontal cup positions in relation to the COR . These programs can also provide the landmark measurements surgeons need . In addition, digital planning was found to be “a reliable and highly accurate method for predicting component sizes and offset” for THA’s performed with two different surgical approaches (direct anterior approach and posterior approach) .
Caution should be exercised when using digital templating for a patient with a higher BMI, one study found that while predictions of the acetabular cup size were unaffected, inaccuracies in predicting femoral component sizing were significant in this population .
Additional training on digital templating software is also needed to take advantage of the full range of options these programs offer. The software is expensive and not available in all hospitals.
Digital templating with acetate overlay
Another common templating approach combines digitally calibrated x-rays with physical acetate templates–merging aspects of both analogue and digital templating. This is also an effective strategy and shown to be much less expensive and faster to perform that using digital software .
This video from Anuwat Pongkunakorn, MD, and Assistant Professor of Orthopedics at the Lampang Hospital and Medical Education Center in Thailand, outlines the steps of templating the acetabular and femoral components with this method.
Landmarking and making measurements when templating
To optimize component placement, it is important to identify landmarks during planning (and intraoperatively) , make calculations that inform placement decisions, and assess a patient’s biomechanics.
Regardless of which method of templating a surgeon uses, a step-wise approach is recommended. Although Scheerlinck suggested the following four-step approach specifically for analogue THA templating, he pointed out that a systematic and standardized approach to this process will improve hip function and outcomes in general .
- Identify anatomical landmarks (the medullary canal, the greater and lesser trochanter, the acetabular roof and the teardrop)
- Assess the quality of the radiograph (femoral rotation, pelvic inclination, and symmetry)
- Identify mechanical references (the original acetabular and femoral rotation center, the femoral and acetabular offset and the leg length and hip length discrepancy)
- Optimize implant positioning to restore hip biomechanics
Figure 3 and Figure 4 demonstrate the anatomical and mechanical landmarks used for accurate templating. Characteristics of these landmarks differ by individual; careful examination of patient-specific traits is important in the planning phase.
Figure 3. Anatomical Landmarks: (1) femoral medullary canal; (2) greater trochanter; (3) saddle; (4) lesser trochanter; (5) acetabular roof; (6) teardrop; (I) obturator foramina; (II) symphysis pubis; (III) sacrococcygeal joint; (IV) distance to measure for pelvic tilt observation.
Figure 4. Mechanical landmarks: (1) hip rotation center; (2) longitudinal axis of proximal femur; (3) femoral offset; (4) acetabular offset; (5) hip length; (6) LLD is calculated as difference between distances 6R and 6L.
Shaikh AH. Preoperative Planning of Total Hip Arthroplasty. 2018. 10.5772/intechopen.76368. https://www.researchgate.net/publication/328797039_Preoperative_Planning_of_Total_Hip_Arthroplasty. Used with permission under CC BY 3.0 License.
Inadequate THA planning can be likened to this quote from Antoine de Saint-Exupéry, “A goal without a plan is just a wish” . Considering that even experienced surgeons make errors in cup placement , the importance of the planning phase cannot be stressed enough because this is when potential problems can be identified and mitigative plans prepared for different situations, should they arise. Bassam Masri, MD, reminds us that,
“The three P’s always apply. Planning. Planning. Planning. No amount of technical skill will compensate for a sloppy plan. The surgeon should always have a Plan A for when the operation goes well, and Plan B and Plan C for when complications occur. Appropriate instruments and implants for the unexpected situation should be available for the rare circumstance when disaster strikes.”
Part III of this article series examines surgical elements of patient-specific THA acetabular cup placement.
This series of articles was created with the support of the following specialists (in alphabetical order):
This issue was created by Word+Vision Media Productions, Switzerland
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