Retrospective Study of Patellar Tracking in an Anatomical, Motion Guided Total Knee Design Adam I. Harris, M.D. & Michelle Ammerman
History: The Total Condylar knee represented a significant advance in the design of and instrumentation used for total knee replacements. Most designs for total knee implants since the introduction of the total condylar knee follow the same basic principles.
Modern Condylar Designs Some are more round (or claim to be) Some are more conforming Some have smoother curves Some are gender specific But they are all...
Basically Condylar Designs
Principles of Surgical Technique Based on the Traditional Condylar Design Rotational position of the Femur is to match the epicondylar axis or Whiteside s line Rotation position of the tibia to align with the junction of the medial 1/3 rds and lateral 2/3 rds of the tibial tubercle
Still Debated Ligament Balancing vs. Measured Resection Mobile Bearing vs. Fixed Bearing designs CR vs. PS designs
Kinematics of Condylar Designs Tend to demonstrate paradoxical motion Tend not to recreate the normal medial pivot (even the rotating platform designs)
All are still basically condylar designs
What about non-condylar designs? Two exist today Wright Medical Advance, Evolution Smith & Nephew Journey BCS, Journey BCS II
Wright Medical Spherical Medial compartment Medial sliding obtained by highly conforming medial compartment Effectively, constraint by the design Rolling in the medial compartment is blocked by the implant
Smith & Nephew Anatomic Design Recreates the normal step of the tibia Recreates the differential condylar thickness Substitutes for both ACL & PCL
Are the rules the same? I would suggest that they are not
Observations with the Journey BCS The tibial base plate is asymmetric The best fit does not always align the center of the tray with the 1/3:2/3 position relative to the tubercle
Observations with the Journey BCS When patellar tracking was not adequate: Downsizing and externally rotating the tray to the traditional position did not help measurably Extensive lateral release did not help much more Downsizing the femur usually fixed the patellar tracking issues
Observations with the Journey BCS Patients tended to tolerate a knee that was looser in flexion than extension well Post operative films suggested excellent patellar tracking in the vast majority of patients
Observations with the Journey BCS The Journey BCS is relatively wide at the sulcus terminalis Overhang was often avoided by downsizing the implant
Migration of Surgical Technique I started downsizing the femoral component so as to minimize or avoid overhang I noted fewer issues intra-operatively with patellar tracking
Quality Improvement Collected data intra-operatively to try to identify the better clinical results Tibial rotational position Femoral measured vs. implanted size Femoral overhang if any Patellar tracking Surgical maneuvers required to get the patella to obey the no thumbs rule
Hypothesis Patellar tracking, both intra-operatively and post operatively is largely independent of tibial rotational position Patellar tracking is dependent on femoral sizing, overhang in particular
Tibial Rotational Position Tibial Rotation initially in 5 categories 1. 1/3:2/3 position or further external 2. Inside the medial border to 1/4 th of the tubercle 3. At the medial border of the tubercle 4. Internal 0-5 mm 5. Internal >5 mm Grouping A. Groups 1 & 2 above B. Groups 3,4,5 above
Radiographic Correlation Merchant view at 6 weeks post op Patellar tilt Patellar displacement
Study Population 254 consecutive knee replacements in 232 patients Each knee evaluated separately in patients with both knees replaced Anterior referencing system One surgeon
Radiographic Measurements Patellar Tilt Patellar Displacement Patellar Thickness Patellar bone - patellar prosthetic angle Femoral condylar - patellar prosthetic angle
Statistics 11 χ² tests 7 correlation coefficients No Bonferroni correction Increased risk of type I error Diminished risk of type II error
Results No correlation between any of the intra-operative measurements and the radiographic evaluation
Trends Observed Those requiring greater intra-operative manipulation to obtain adequate patellar tracking tended to track less well post-operatively Those with greater patellar tilt post-operatively tended to have more patellar tilt pre-operatively Clinically, patients seemed to tolerate a flexion/extension gap mismatch, loose in flexion, better than femoral overhang
Weakness of the Study Prior experience created a variation in the surgical technique Limiting femoral overhang intra-operatively limited the population with overhang, and may have obscured any effect
Other Observations Others have reported a high incidence of iliotibial band traction syndrome with this implant. In my roughly 500 patients only one carries this diagnosis. Artificially externally rotating the tibia in a medial pivot design will make the lateral femoral condyle more prominent during mid flexion My anatomic placement of the tibial component may be responsible for the nearly zero incidence of this problem in my population
Conclusions Hypothesis 1: Patellar tracking is independent of tibial component position Supported
Conclusions This in vivo study confirms both the cadaver and computer simulation studies done with this implant The tibial component may, and possibly should be positioned for maximum coverage or to match the natural anatomy rather than be placed in the traditional position
Conclusions Hypothesis 2: Patellar tracking is dependent on femoral sizing, and overhang in particular Neither supported nor refuted The observations that prompted the study also prompted a change in surgical technique The number of knees with overhang was small and within that group, the overhang itself was small
Conclusions Iliotibial traction band syndrome may be less of an issue of the design of the implant than of the technique for implantation Forcing non-anatomic positioning of anatomically designed components probably creates nonanatomic forces
Conclusions Optimal outcomes for prostheses designed to recreate the native anatomy may not require the same rules as the traditional condylar designs The exact rules that should apply to anatomic designs are not yet completely understood and should be the subject of further investigation
Thank You Adam I. Harris, M.D. & Michelle Ammerman
Type to enter text Retrospective Type to enter text Study of Patellar Tracking in an in Anatomical, Motion Guided Total Knee Design an Adam I Harris, M.D., & Michelle Ammerman Abstract: Poor patellar tracking is an identified source for post operative pain. In condylar designs, patellar tracking is dependent upon the rotational position of the components. Early experience with the Journey BCS (Smith and Nephew, Memphis, TN, USA) suggested that this guided motion implant was not subservient to the same dictates. This study was conducted to determine the relationship between the rotational position of the tibial tray and the patellar tracking as well as identify other factors that might play significant roles. The hypothesis was that patellar tracking is independent of tibial tray rotation. Intraoperative data was collected prospectively on 254 consecutive Journey BCS total knee arthroplasties. Included were: the size and position of the tibial component, the measured size of the femur, the implanted size of the femoral component, overhang of the femoral component if any, and intraoperative steps needed to obtain proper patellar tracking. Radiographs were reviewed retrospectively for patellar angulation and displacement. No correlation (Chi square or linear regression as appropriate) was found between the rotation of the tibial component and the quality of the patellar tracking. There was some suggestion that overhang of the femoral component most profoundly and adversely affected patellar tracking. The observation that prompted the study also prompted a change in technique that may have concealed any effect of femoral overhang. There is limited information on variance of these parameters, but the study appears to be otherwise adequately powered. The study suggests that in Journey BCS where contours mimic the normal native knee, the tibial component can, perhaps should be placed in line with the normal anatomy rather than the same position as previous designs. It further suggests that iliotibial band traction syndrome is instead a a result of non anatomic positioning of an anatomic surface. Background: Total knee arthroplasty (TKA) is accepted as a top quality of life surgery 5. Many authors report that postoperative complaints are centered upon issues relating to the patella. The major complications include fractures and frank dislocation 4,6,7,10. In addition, another population has complications with poor patellar tracking. Although it is often considered a minor complication, it can lead to significant discomfort. The patellar can tilt or move laterally. In both cases pain that may be caused by rubbing of the remaining patellar bone upon the femoral implant may be reported by the patient 1,10. Abnormal forces across a patellar component that tracks poorly can result in early implant failure 4. In many artificial knees the rotational position of the femoral or tibial component, relative to each s long axis, has a major impact upon the tracking of the patella 2. The Journey BCS knee (Smith & Nephew, Memphis) is different in its design from most total knee implants. Traditional femoral components have equal femoral condylar thickness and equal tibial plateaux thickness neither of which is anatomically correct. By contrast, the Journey BCS design recreates the natural contour of both of the tibial plateaux as well as the natural "step" pattern. At the same time, the lateral femoral condyle is thinner than the medial femoral condyle in order to match the normal pattern. By so doing, the normal pattern of differential rollback of the femoral condyles is restored. There is an asymmetric tibial baseplate design. This allows for better bone coverage, and support of the tibial component. Optimal bone coverages though, comes potentially at the expense of the rotational position of the component. Dr. Harris early experience with the implant suggested that the rotational position of the tibial component was less important than avoiding overhang of the femoral component in order to obtain proper intra-operative patellar tracking. This would allow the surgeon to select the tibial component that had the greatest overall support as well as the most contact with the cortical rim.theoretically minimizing tibial subsidence and loosening. This is also consistent with the anatomic design of the implant, as the orientation of the center of the native tibial spine generally falls internal to the tibial tubercle. A sample of near perfect patellar tracking. Methods and Materials: This study is a review of 254 consecutive knee replacements in 232 patients who had at least one set of outpatient radiographs 6 weeks of more after the index surgery. Intraoperative data was collected prospectively, while radiographs were analyzed retrospectively for this group. Their surgeries ranged from February 2010 to June 2012. Candidates were selected upon two factors: they received a Journey BCS and that their total knee arthroplasty had been performed by Dr. Harris. All surgeries were performed by a single surgeon to create consistency in procedure and technique. Patients with bilateral surgeries were included in the study. Each knee was evaluated separately. The surgical technique conformed to standards in the industry. All attempts were made to maintain the normal joint line, and rotate the femoral component to match the epicondylar axis. Soft tissue releases were performed so as to obtain equal flexion and extension gaps. The goal was to have no more than 1 mm difference between the tightest and the loosest corner. Laxity was measured with a calibrated plastic feeler gauge designed to match the curvature of the femoral condyles. The only two variations were that the tibial component was selected so as to maximize coverage of the tibia without overhang and that if needed, the femoral component was downsized so as to minimize or eliminate overhang. As this group progressed, small mismatches between the flexion and extension gaps were accepted more readily so as to avoid femoral component overhang. The postoperative radiograph of choice was obtained 6 weeks after the surgery. However, due to image quality or positioning subsequent radiographs were used as needed. In total 254 patients x-rays were examined. 232 patients received one or more Journey BCS total knee arthroplasty. The patellar views used for this study were taken in the position described, by Merchant 11. The measurements used for this experiment were: the patellar tilt consisting of the the femoral bone prosthesis angle (FBPA),the patellar prosthesis bone angle (PPBA), the patellar tilt (also the combination of the FPBA and PPBA), the patellar displacement (PD), as well as the patellar thickness. All measurements were done digitally with the FDA cleared version of OsiriX. All radiographic measurements were made by a single independent observer, and only later correlated with intra-operative data. The manufacturer's recommended femoral component size was based on the anteroposterior dimension of the distal femur. Measurements between sizes, for the Journey BCS always lead to selection of the next smaller implant. For 76 patients, the final implants were an additional size smaller so as to minimize or eliminate overhang. Antero-posterior positioning was based on an anterior referencing guide. The tibial component was selected so as to maximize the coverage of the cut surface without overhang. The tibial component was externally rotated to the maximum extent possible within these constraints. The rotational position of the tibial component relative to the tibial tubercle was recorded. Five positions were identified. The center of the tibial component was listed as being 1) at or external to the junction of the medial and central third, 2) between the medial border of the tibial tubercle and one quarter of the width of the tubercle, 3) at the medial border of the tubercle, 4) internal to the medial border up to 5 mm, or 5) more than 5 mm internal to the medial border. As the number of patients falling into some of these groups was small, they were also pooled into two groups, with group A pooling groups 1 and 2 above, while group B consists of the remainder. Femoral overhang, if any, was measured. Additional surgical techniques, if any, required to obtain excellent intra-operative patellar tracking was recorded during surgery. Various statistical analyses were used to determine any correlation between any of the intra-operative measurements and radiographic patellar tracking. χ² tests were performed comparing the rotation of the tibial component, the surgical techniques required to obtain proper intraoperative patellar tracking, and the radiographic analysis of the patellar tracking. The potential relationship of femoral overhang to patellar tracking was analyzed two ways. First the amount of femoral overhang was compared to the lateral and the vertical displacement, and patellar tilt. Second, the presence or lack of overhang was compared to the lateral and vertical displacement, the patellar displacement, the patellar tilt and the additional surgical techniques required in order to obtain proper patellar tracking. Femoral sizing was evaluated relative to patellar tracking as well. Femoral sizing was divided into two groups. One where the size of the implant was within one size of the measured size, and the other where the femoral component implanted was more than one size smaller than the AP dimension of the femur. This was compared to the techniques requisite to obtain proper intraoperative patellar tracking as well as the patellar tilt and displacement. Finally, multiple χ² tests were performed between patellar displacement (lateral and vertical) patellar tilt, surgical techniques required, tibial rotational position. Tibial position was both analyzed in the five groups as described above, as well as in two groups to potentially find a statistically significant relationship. For all tests, a p< 0.05 was deemed statistically significant. Avoiding the use of the Bonferroni correction increased the risk of a type I error. As the hypothesis was that there is not a correlation between tibial rotational position and patellar tracking, the analysis was performed so as to minimize the chance of a type II error. Multiple regressions were also analyzed between: all of the radiographic measures of patellar tracking. After initial statistical analysis any patient with a patellar displacement greater than 5 mm was reexamined. The study held to a higher standard than Chan 2 Their preoperative films were examined for the presence of patellar tilt or displacement. Any abnormal patellar position (tilt or translation) was marked as positive. If a patient did not present with patellar displacement or tilt they were marked with none apparent. Those with measurable, but not more than 5 mm were assigned slight. A sample of no so perfect patellar tracking. Results: Of the 11 χ² tests performed none obtained statistical significance. They all had p values greater than 0.30. Of the 7 correlation tests none presented with significant correlation all presented with minimal. As hypothesized all tests comparing tibial rotational position to intraoperative or post operative patellar tracking showed no statistical significance. The presence or absence of femoral overhang was also not correlated with radiographic patellar tracking. Similarly, undersizing the femoral component was not associated with poor intraoperative or radiographic patellar tracing. Tests relating patellar tilt to patellar displacement (both χ² and correlations) were also not signficant. (p>0.95 R 2 =0.2813). There was a small trend towards a correlation between tilt and displacement. A tighter correlation between these two measures of poor tracking was expected. Exactly why only such a loose correlation was observed is not clear from this data. There was also a trend for those patients who required more extensive techniques in order to obtain proper intraoperative tracking to have suboptimal post operative tracking. (p=0.33 for patellar displacement and p=0.037 for patellar tilt). Conversely, those who required no special attention intraoperatively are likely to demonstrate good postoperative tracking. The design of the study though may have prejudiced against identifying a correlation between overhang and patellar tracking. Intraoperative trials demonstrating more than 3 mm of overhang (the difference between sizes) were downsized before patellar tracking was evaluated even though some of these implants still demonstrated femoral component overhang. The preoperative vs. postoperative data was the most indicative. 64 patients had a combined displacement falling outside the +/-5 mm range. 55 of these patients (86%) presented with preexisting maltracking. Thus suggesting that post operative patellar tilt is most closely related to preoperative tracking. Patellar Displacement FPBA Patellar Tilt Intraoperative tracking (excellent, vs. work needed) Tibial Rotational Position Patellar Femoral Displacement Overhang Intraoperative tracking (excellent, vs. work needed) Tibial Rotational Position - p = 0.95 - - R 2 = 0.002 p = 0.96 - - R 2 = 0.144 p = 0.92 p = 0.91 p = 0.91 p = 0.99 p = 0.99 - - p = 0.94 - p = 0.98 - A sample of the statistical tests run on this data Discussion: Two hypotheses were tested in this study. The first was that the radiographic patellar tracking was not related to the rotational position of the tibial component. This hypothesis was supported. The second hypothesis was that radiographic patellar tracking was most closely related to femoral component overhang. This hypothesis was not supported. Other observations derived from this study were first that there was a weak trend for those patients who required extra releases or other surgical interventions in order to obtain proper intraoperative patellar tracking to subsequently not track as well despite these interventions. One would expect a correlation between patellar tilt and patellar subluxation as both are measures of patellar instability. There was a trend observed towards such a relationship. The combined patellar tilt angle and patellar displacement were weakly correlated. (R 2 =0.144) This study was probably also underpowered to make such a determination. The strongest trend identified in this study was the association of preoperative patellar maltracking to post operative patellar maltracking. Eighty-six percent of patients with post operative patellar displacement greater than 5 mm presented with some patellar maltracking in their preoperative studies. This was consistent with other studies although they implicated other factors that were not addressed in this study 3,10. This study also held a higher standard for acceptable patellar tracking on post operative radiographs. Many of the patients identified with suboptimal tracking might have been considered acceptable in other studies. Doing so may have increased the odds of finding a true positive correlation. Despite this, only trends were identified. Evaluation of pre operative patellar tilt was only performed on those with post operative patellar tilt. Therefore, no information is available for those whose pre operative patellar tilt may have been corrected by the surgery, but there is some information on the pre operative status of those with patellar tilt postoperatively. All of the tests performed in this study comparing tibial rotation and patellar displacement or tilt were statistically insignificant. This suggests that the rotation of the tibial component is independent of patellar tracking. One weakness of this study was that the observations that lead to the study also lead to a change in surgical decision making. While not formally measured in any manner, patients seemed to tolerate greater laxity in flexion than in extension better than femoral overhang. Thus, the size of the group with measurable overhang was small, and within that group, the overhang itself was small. Thus, it was not possible to prove or disprove the hypothesis that patellar tracking was dependent upon the extent of femoral component overhang. Further, aggressive downsizing of the femoral component and more anatomic positioning of the tibial tray of the Journey BCS design may account for the paucity of iliotibial band traction syndrome in this study compared to the more than 7% previously reported. 9 Data probably does not exist that could prove or disprove this point as well. In as much as the second generation design is less wide and is less aggressive at forcing normal rollback during flexion, if the incidence of iliotibial traction band syndrome drops with the new design, the true etiology of the iliotibial band traction syndrome may never be elucidated. Lastly, there is some support in this study for the previously reported correlation between pre operative patellar tracking and post operative patellar tracking. Conclusions: The Journey BCS total knee design is significantly different from that of most available implants. The well established guidelines for optimal component positioning elucidated over the past several decades for the basic condylar design may not all apply to this implant. This study suggests that the tibial component may (and perhaps should) be selected to maximize coverage of the tibia and to match the natural anatomy with less regard to the rotational orientation than with conventional designs.