Minimally invasive surgery (MIS) is becoming increasingly popular. Supporters claim that the main advantages of MIS total hip replacement (THR) are less pain and a faster rehabilitation and recovery. Critics claim that safety and efficacy of MIS are yet to be determined. We focused on a biomechanical comparison between surgical standard and MIS approaches for THR during the early recovery of patients. A validated, parameterized musculoskeletal model was set to perform a squat of a 50th percentile healthy European male. A bilateral motion was chosen to investigate effects on the contralateral side. Surgical approaches were simulated by excluding the incised muscles from the computations. Resulting hip reaction forces and their symmetry and orientation were analyzed. MIS THR seemed less influential on the symmetry index of hip reaction forces between the operated and nonoperated leg when compared to the standard lateral approach. Hip reaction forces at peak loads of the standard transgluteal approach were 24% higher on the contralateral side when compared to MIS approaches. Our results suggest that MIS THR contributes to a greater symmetry of hip reaction forces in absolute value as well as force-orientation following THR. © 2014 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 32:1680–1687, 2014.
Primary total hip replacement (THR) is one of the most performed but increasingly costly operations within orthopaedic surgery. Although THR survivorship has markedly increased within the last decade, early failure after THR is still reported as high as 24%. In this context, the use of minimally invasive surgery (MIS) in THR has become more prevalent. Proponents argue that MIS techniques reduce soft-tissue trauma, decrease postoperative pain and blood loss, speed recovery, and reduce the hospital stay compared with THR using a standard approach. However, critics claim that poorer visualization of the joint in MIS may lead to higher complication rates so that safety and efficacy have yet to be determined.
Clinical studies comparing MIS and standard THR using intraoperative data and common clinical scores have been done, but biomechanical analyses are still rare. With the rise of computational power, new numerical models of human biomechanics have been developed. More recently, prosthetic hip reaction forces (hrf, —also called hip contact forces—hcf) can be computed in silico for biomechanical analyses after THR, which would be difficult to perform in vivo. In this context, symmetry indices are often used as outcome measures since an increase in symmetry leads to a more equally distributed load scenario in the artificial hip joint. Therefore, symmetry of the hrf estimates how much the specific surgical approach influences the opposite (untreated) side. The hip reaction angles (hra—orientation of hrf) estimate if critical edge (or rim) loading occurs during a certain motion. A knee bend can also be compared to the “sitting down” motion that is known to be the third most common motion that THR patients perform during their daily life.
In this context, analyzing prosthetic hrf during a squat is of special interest. First, this exercise is a known risk factor for impingement and/or edge (rim) loading, causing higher wear rates. Second, postoperative bone loss and implant fixation after THR can be countered by proper rehabilitation programs that include “non-impact-exercises” also capable of increasing bone mineral density (BMD). Such mechanical stimuli, as induced on the femur by the squat, are the triggers for bony in growth of cementless THRs.
Therefore, we compared the biomechanical mechanisms of common MIS and standard surgical approaches for THR using a simulation of a squat motion, focusing on the early postoperative influence of tissue trauma regarding: absolute hip reaction force (hrf), symmetry of hrf, and orientation of hrf. We hypothesized that the tissue trauma caused by MIS THR is less influential on the hip reaction forces in an early postoperative stage.
MATERIAL & METHODS
Approaches were divided (Table 1) into conventional and minimally invasive (MIS) approaches. Categories for classification were the patient position, skin incision, and incised/detached muscles.
|Approach||Patient Position||Incision||Muscle and/or Tendon Detached and/or Incised; in Order of Intervention|
|Conventional||Anterior ||supine||from middle of iliac crest||- tensor fascia latae (deep for visualization)|
|to anterior superior illiac spine||- gluteus medius|
|- rectus femoris|
|Anterolateral ||supine||from 2.5 cm posterior and distal to the anterior superior iliac spine||- tensor fascia latae|
|to 5 cm distal to the greater trochanter||- gluteus medius|
|- vastus lateralis|
|Lateral ||supine||from 5 cm proximal to the greater trochanter||- tensor fascia latae|
|to 5–6 cm below it (along the femoral shaft)||- gluteus maximus|
|- gluteus medius|
|- vastus lateralis|
|Posterior ||lateral||from 10 cm from posterior iliac spine directed over trochanter||- tensor fascia latae|
|to 10 cm below trochanter along shaft||- gluteus maximus|
|- obturator externus|
|- gluteus medius|
|- quadratus femoris|
|Transgluteal ||supine||from 5 cm proximal to the greater trochanter||- tensor fascia latae|
|to 5–6 cm below it (along the femoral shaft)||- vastus lateralis|
|- gluteus medius|
|- gluteus minimus|
|Minimally-invasive (MIS)||Anterior ||supine||from 2 cm distal and lateral to the ASIS||- tensor fascia latae|
|to 2 cm anterior to greater trochanter (with 30° flexed hip)||- sartorius|
|Posterior ||lateral decubitus||from midway greater trochanter||- gluteus maximus|
|to 6–8 cm direction of anterior superior illiac spine||- piriformis|
|Anterolateral ||lateral decubitus||from midway greater trochanter to 6–8 cm direction of anterior superior illiac spine||- no tendons or muscles are cut or detached (muscle trauma is kept to an absolute minimum)|
A previously validated musculoskeletal model (MM)[25, 26] and a commercial software package (AnyBodyModeling System 6.0.1, Aalborg, Denmark) were utilized to compute the hrf. The MM resembles a 50th percentile healthy European male in terms of body mass (75 kg), height (180 cm), fat-percentage (21%), and muscle geometry.[27, 28] The muscles were parameterized using the mechanical Hill-Type muscle model; the tendons were calibrated accordingly.[29-31] The masses and lengths of the body parts (legs, arms, trunk, and head) were derived from anthropometric measurements. Ligament properties were based on a cadaver study.
Using full body inverse dynamics, the MM (Standing model, AMMR 1.5) performed a deep squat (maximum knee flexion: 80°; maximum hip flexion 80°) as a motion known to be prone to impingement and dislocation (Fig. 1).[13, 34] Normal kinematics were chosen since no data were available of a squat motion two weeks after surgery and patients are instructed to avoid such movement. The motion was derived as 8°/s knee flexion and 8°/s hip flexion. Total simulation time was 10 s with 100 computation steps.
The time-dependent muscle activity was determined by a cubic optimization scheme:
where G is the objective function to estimate muscle activation, f is the muscle force vector (fi is the ith element), and N is the normalizing factor (muscle strength). G is to be minimized while the boundary conditions must be satisfied (equilibrium fulfilled, muscles can only pull).
We checked our computed hrf and the computed ground reaction forces as retrieved by each MM against measured hrf from instrumented implants. The HIP98 dataset is publicly available (http://www.orthoload.com/) and generally accepted to validate MM. We used all available data, measured in four patients during a knee bend and sitting down, resulting in 28 experimental datasets.
Modeling the Surgical Approach
The muscles incised during a specific surgical approach were excluded from the simulation to mimic the influence of surgical approach on hip biomechanics early after surgery (Fig. 2). This was done in a “worst-case” scenario and to provide baseline data for modeling the influence of muscle trauma after THR. Closest to the reference model would be the anterolateral MIS approach (MicroHip® - MH), where only a minimum of muscles/tendons are detached or incised. A femur is displayed in Figure 2 with the respective muscle insertion and origin points. The respective muscle tissue (muscle and tendon) that was excluded from simulation was suppressed in the figure.
Only the muscles on the right leg were influenced; the contralateral muscles were left unchanged. For each approach, one model was developed resulting in nine MMs including one reference model without influenced muscles. The muscular damage patterns in our MM represent a situation within the first two weeks after THR. We chose a bilateral movement to investigate hrf changes on the contralateral side.
Post-processing of the MM results was performed using MATLAB (MATLAB Release 2011b, The MathWorks, Inc., Natick, Massachusetts, United States.) The hrf symmetry index (symmetryhrf) was computed by dividing the absolute values of the hrf on the treated side (hrfop) by the absolute values of the hrf on the not-treated side (hrfnoop):
The hrf-orientation (for inclination and anteversion) symmetry index (symmetryhra) was computed by dividing the hip-reaction-angles (hra) on the treated side (hraop) by the absolute values of the hra on the untreated side (hranoop):
Orientation of hrf at every time step ti was quantified in the radiographic coordinate system.
The comparison of our computed hrf to the experimental data from instrumented implants showed good agreement in terms of trends and absolute values (Fig. 3).
Absolute Hip Reaction Force
On the left hand side of Figure 4, the healthy left leg is displayed. The results for the hrf on the intervention side (right leg) are displayed on the right. The maximum hrf appears in all models at the end of the knee bend (10 sec or 100% knee bend). Minimum values for the hrf during the knee bend can be found in the Smith–Petersen model (1.46*bodyweight), where the greatest values for the hrf can be found for the Bauer model (2.1*bodyweight). The hrf retrieved from the MIS models were closer to the reference model (without tissue trauma) when compared to the conventional approaches. While the Bauer approach leads to higher hrf, the relative deviation to the reference model is ∼7%. The Smith–Petersen approach led to smaller hrf (75% of reference). Closest to the reference model were the hrf retrieved by the Micro Hip model. On the contralateral side, MIS approach models altered the hrf by a maximum of 0.5%; however, the hrf retrieved from the conventional models were altered from 6% (Smith–Petersen model) up to 24% (Hardinge model), when compared to the reference model.
Symmetry of Hip Reaction Forces
Figure 5 displays the symmetry index for all surgical approach models over the entire knee bend. The greatest assymetries were found in the Bauer model (maximum) and in the Smith-Petersen Model (minimum). At peak loads during the squat, the biggest deviation of symmetry was found in the conventional models, whereas the MIS approach models were closest to the reference model. The conventional models showed an increasing asymmetry with increasing flexion angles of the knee and hip. In the MIS models, asymmetries were also apparent, but they did not increase during the squat (Figure 5).
Orientation of Hip Reaction Forces at Peak Loads
Table 2 lists the orientation of the hip-reaction-force vector in the radiographic coordinate system for every modeled surgical approach including the reference model. Maximum deviations from the reference model were found in the Moore model with a force inclination of 129° and a force anteversion of 163°. In general, the MIS approaches were closer to the reference model than the conventional approaches. The maximal symmetry deviation (Table 2) was also found in a conventional approach model (inclination angle—Hardinge model). This was limited to a maximum of 9% deviation. Symmetry deviation of MIS approaches did not exceed 1%. The hra at peak loads of the MIS models were close to the reference model, whereas the hra as retrieved from the conventional models were increased, and even influenced the force orientation on the opposite leg (closed kinematic chain).
Changes in Muscle Force and Activity
The change of maximum muscle and maximum muscle activity force that was induced by a specific surgical approach is shown in Figure 6. The muscles are divided in hip function groups: hip abductors (gluteus medius, gluteus minimus, sartorius, tensor fascia latae), hip flexors (psoas major, psoas minor, iliacus, rectus femoris, sartorius, tensor fascia latae, pectineus, adductor longus, adductor brevis, gracilis), hip adductors (adductor brevis, adductor longus, adductor magnus, adductor minimus, pectineus, gracilis, obturator externus), and hip extensors (gluteus maxmimus, biceps femoris caput breve, semimembranosus, semitendinosus). The surgical approach is shown on the horizontal axis. On the (first) left vertical axis, the maximum computed muscle activity is displayed; on the right (second) vertical axis, the maximum computed muscle force is displayed. On the left of each plot, one can find the reference values for comparison. The biggest change was found for the conventional surgical approaches for the hip abductors. MIS approaches showed little influence on the muscle force and activity. While the Moore approach led to great differences in hip abductors, flexors, and extensors, muscle activity and force was close to the reference model in terms of extensors.
We compared the most common MIS and standard surgical approaches for THR using a biomechanical simulation of a squat motion, focusing on the early postoperative influence of tissue trauma regarding absolute hip reaction force (hrf), symmetry of the hrf, and the hrf orientation. We hypothesized that the influence of tissue trauma caused by MIS THR is less influential on the hrf. In general, MIS approaches appeared to have less influence on the target parameters, even in this “worst-case” and baseline investigation.
Our study has several limitations. First, the impact of muscular trauma was modeled on a theoretical basis. The assumptions of muscle damage were based on the description of the surgical approaches in the literature. The level of trauma also depends on instruments, intraoperative soft tissue management, surgical experience, and particularly on patient anthropometry (age, weight, BMI, muscular contractures). Second, modeling the influence of surgical approach by setting the muscle strength to null cancels other patient-specific influences, such as reduced postoperative neuromuscular activity due to pain. Such factors may further influence the muscular activation and therefore the hrf. Future research on THR patients may give better insight using electromyography to evaluate muscle activation in conjunction with motion capture to assess patient-specific motions. However, our musculoskeletal model also offers various advantages. First, it is possible to only alter muscle properties, without the results being subject to biological variance, which is a challenge when researching biomechanics or altered muscle activation patterns. The parameterized approach of the knee bend ensures that the model always performs the same squat. Data retrieved by measuring equipment may be subject to errors and deviations that can be canceled out using a computational approach. Our target parameters (hrf, hra, symmetries) provide a differentiated view into THR biomechanics. The data set used for the MM in this study is based upon data collected by Klein Horsmann et al.[33, 38] as retrieved by cadaveric studies. Crucial modeling parameters such as segment lengths, segment masses, muscle moment arms, and muscle fiber pennation angles are consistent throughout the model.
In the context of this study, tissue trauma as caused by a MIS approach was less influential on the hrf as trauma caused by a standard approach for THR. The absolute hrf on the influenced side did decrease noticeably for the Smith–Petersen (75% of reference) and the Moore (84% of reference) Models during the squat. Differences of the remaining models to the reference model (MIS and conventional) seem clinically irrelevant with a range from 2% to 6%. This is confirmed by the computed muscle force and muscle activity. The conventional approaches showed the greatest deviations when compared to the reference model. Incising the hip flexor led to great deviations (Smith–Petersen) and a wide range of posterior muscle groups (Moore) seemed also to greatly influence the postoperative biomechanics. Muscle trauma as caused by MIS caused smaller deviations to the reference model in general and when compared to conventional approaches. Tissue trauma as caused by conventional approaches also influenced the hrf on the contralateral side (up to 24%), which was confirmed by the symmetry index. The decreasing symmetry index during the motion shows that conventional approaches are also more influential during the motion, reaching the biggest symmetry deviation at peak loads. The MIS models resulted in a more symmetrical load distribution during the squat. Even if asymmetries were apparent, they were smaller compared to conventional approaches and they also remained within the same magnitude. The maximum deviations in hra (9%) seem clinically irrelevant. The force orientation suggests that rim-loading did not occur in either model. Closest to the reference model would be the MicroHip® approach, which is described with a minimum of muscle trauma.
This study adds a novel perspective on the biomechanics of the hip joint early after THR and provides baseline measurements for further studies. Minimally invasive techniques have the potential for an improved biomechanical situation in the early postoperative situation after THR.
We thank Peter Waller, University of Regensburg, British Studies for the help with the revision of this manuscript. This study was funded by the Ostbayerische Technische Hochschule Regensburg.