Does the severity of preoperative varus deformity influence postoperative alignment in both conventional and computer-assisted total knee arthroplasty?

Registration of Proximal Tibial Centre May Need to be Selectively Lateralized to Avoid Coronal Malalignment in Digitally-Assisted Knee Arthroplasty

PURPOSE

Placement of prosthetic components and limb alignment can be adversely impacted by errors in identification of registration points in digitally assisted knee arthroplasty. The purpose of this study was to trigonometrically analyze the impact on the accuracy of coronal tibial component and limb alignment of discrepancy between the registered midpoint of the proximal bony tibia vis-à-vis the radiographic medio-lateral center of the lateralized final tibial implant in certain varus-deformed knees.

METHODS

We analyzed trigonometrically the angular difference θ in coronal alignment of tibial component and hip-knee-ankle angle resulting from lateral translation of the medio-lateral center of the final tibial implant, with respect to the initial registered midpoint of the proximal bony tibia. θ was calculated using the equation tan( θ ) = d/t, where d is the numerical distance between the registered point and the center of the final tibial tray position and t is the length of tibia distal to the resection.

RESULTS

One degree of varus malpositioning of the tibial component and the limb can occur for every 6.3 mm of lateral translation of the tibial implant with respect to the midpoint of the proximal bony tibia of average reported length (372 mm). The magnitude of this error is inversely proportional to tibial length and the patient's height.

CONCLUSIONS

Coronal malalignment of the tibial component and the limb is directly proportional to the amount of lateral translation of the tibial tray with respect to the registered midpoint of the proximal bony tibia and inversely proportional to the patient's height. This may occur in cases with tibial extra-articular deformity, those undergoing reduction osteotomy, downsizing of the tray, or inadvertently during tibial preparation. The error can be avoided by initial registration of the tibial midpoint more laterally by templating, re-registering the new center of the resected tibial surface or osteotomy for extra-articular deformity.

Tibial Tray with a Stem: Does It Have Any Role in Primary Cemented Total Knee Replacement?

Over many decades, total knee replacement (TKR) has become the ideal treatment option for advanced arthritis. Many designs were introduced to increase the stability of the tibial tray, hence the longevity of the prosthesis. This retrospective study was performed on 168 patients who received NexGen cemented primary total knee either with standard tibial tray (group A) or tibial tray with an intramedullary stem (group B) between May 2008 and May 2017. We reviewed all preoperative and postoperative clinical and radiological data retrospectively. In addition, a prospective clinical and radiological reassessment was done. Our aim was to answer the following questions: (1) Is there any difference between both groups in regard to clinical and radiological results? (2) Is there any difference in the revision rate? (3) Is there a role of using stemmed tibial tray in primary TKR? Better results were recorded in obese and severely obese patients having stemmed cemented tibial tray and so for patients with marked and severe preoperative varus angle. Our recommendations are to use cemented tibial tray with a stem in complicated primary surgery without fear of adverse effects on short and intermediate terms of follow-up.

Degree of coronal alignment correction can't predict knee function in total knee replacement

BACKGROUND

Whether neutral alignment brings better clinical outcomes is controversial. Consideration of the preoperative knee condition of patients and some limitations of previous studies, we suggested that other index may be more important than a generic target of 0° ± 3° of a neutral axis to reflect changes in coronal alignment after total knee replacement (TKR). The purpose of this study was to explore the relationship between alignment and functional outcome with a new grouping method and the concept of correction rate.

METHODS

The study included 358 knees, the mean follow-up period was 3.62 years. A new grouping method was adopted to divide patients into three groups based on the degree of correction of mechanical femoral-tibial angle (MFTA): under-correction (n = 128), neutral (n = 209) and over-correction (n = 21). Hospital for Special Surgery (HSS) score were compared among the 3 groups (ANOVA with or without LSD t-test). In addition, we also attempt to further explore whether the concept of correction rate can predict postoperative functional score (Simple linear correlation analysis).

RESULTS

HSS score showed significant improvement in all groups. There was no difference in HSS score (88.27 vs 88 vs 85.62) (p = 0.88) or incremental scores (26.23 vs 25.22 vs 22.88) (p = 0.25) based on the postoperative alignment category for the degree of correction of MFTA at the last follow-up. The correlational analyses also didn't show any positive results (r = -0.01 p = 0.95, r = -0.01 p = 0.97, r = 0.11 p = 0.15, r = 0.01 p = 0.90).

CONCLUSION

Categorization of optimal coronal alignment after TKR may be impractical. But we still believe that the concept of correction rate and new grouping method are worthy of research which can reflects the preoperative knee condition and the change of coronal alignment. Perhaps it can be better used in TKR in the future.

LEVEL OF EVIDENCE

III.

The Use of Computer Navigation and Robotic Technology in Complex Total Knee Arthroplasty

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The potential benefits of computer-assisted surgical (CAS) navigation and robotic total knee arthroplasty (TKA) systems, such as increased reliability of restoring the mechanical axis, fewer outliers, more rapid hospital discharge, less physical therapy requirements, decreased blood loss, and decreased revision rates, have led to their application not only in primary cases but also in complex cases such as preoperative deformity and revision.

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Early evidence demonstrates that CAS navigation may help to improve alignment in complex cases of femoral and tibial deformity and in cases of femoral bowing.

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Data regarding deformity correction with robotic systems are similar to CAS navigation with regard to alignment, but are more limited. There are also scant data regarding revision cases and cases of previous intramedullary canal instrumentation.

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Concerns remain regarding cost, learning curves, and operative times. There are potential long-term cost savings associated with a decrease in revisions and readmissions that require additional investigation.

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Early evidence for the use of these emerging technologies for deformity correction and revision cases is promising, but their impact on long-term functional outcomes remains to be demonstrated. Additional well-designed comparative studies are warranted.

Effectiveness of Using Photographs of the Change in Standing Posture on Postoperative Patient-Reported Satisfaction and Quality of Life

Dissatisfaction is still an important problem in a small but important group of patients who undergo total knee arthroplasty. This study was designed to evaluate the effectiveness of showing patients the change in their standing posture, before and after total knee replacement, using standing photographs (anterior, posterior, and lateral view), on improving self-reported quality of life and satisfaction. Full-length lower extremity radiographs and standing photographs were obtained prior to total knee replacement and 6 months after surgery in the study group. In the control group, radiographs and photographs were not obtained. The hip-knee-ankle angle and mechanical axis deviation were compared between the two limbs and two groups. The changes in the following outcome measures were evaluated from baseline to 6 months after surgery: Short Form-36 Survey, Knee Injury and Osteoarthritis Outcome Score, Western Ontario and McMaster Universities Osteoarthritis Index, Oxford Knee Score, and Knee Society score. Postoperative measures were also compared before and after showing patients their standing photographs, and between two groups. The study and control groups included 71 and 44 patients, respectively, with unilateral or bilateral knee osteoarthritis (Kellgren-Lawrence Grade 3 or 4), with a mean age of 66 years. All patients had a varus malalignment, with significant improvement in postoperative limb alignment (p < 0.001). All self-reported outcome measures improved after surgery (p < 0.05), with the exception of mental health in the Short Form-36. Self-reported scores further improved after patients seeing their standing photographs (p < 0.05), with the exception of mental health and social role functioning. There were also significantly improved scores in the study group than control group after showing photographs (p < 0.05). Showing patients the pre-to-postoperative change in their standing posture might be an easy-to-administer method to improve patient satisfaction with the outcome of total knee replacement and self-reported quality of life. The Level of Evidence for this study is four.

Impact of preoperative varus deformity on postoperative mechanical alignment and long-term results of "mechanical" aligned total knee arthroplasty

PURPOSE

Preoperative varus deformity may affect postoperative mechanical alignment and outcomes of total knee arthroplasty (TKA). In this study, we aimed to determine whether (1) greater degrees of preoperative varus deformity increase the incidence of varus-aligned TKAs and (2) residual varus alignment improves the long-term survival rate in TKAs with preoperative varus deformity.

METHODS

We retrospectively reviewed 905 primary TKAs for varus-type osteoarthritis from November 1998 to June 2009. The mean follow-up was 8.23 years (±3.47 years). We measured pre- and postoperative mechanical hip-knee-ankle axis angle (HKA) on full-length standing radiographs to assess the severity of preoperative varus deformities, defined as mild (0°<HKA≤5°), moderate (5°<HKA≤10°), severe (10°<HKA≤15°), and very severe (HKA>15°). Postoperative alignment was divided to four groups: valgus (HKA<-3°), neutral (-3°≤HKA≤3°), mild varus (3°<HKA≤6°), and severe varus (HKA>6°). We performed survival analysis in all cases for mechanical failure.

RESULTS

Varus-aligned TKAs increased proportionally to increasing preoperative varus deformity (p<0.001). The survival rate showed no significant difference according to the preoperative varus deformity (p=0.147). However, the survival time for postoperative neutral alignment (16.13±0.10 years, 95% CI 15.94-16.33 years) was longer than the survival time for mild varus (15.32±0.35 years, 95% CI 14.64-16.00 years) and severe varus (13.38±0.53 years, 95% CI 12.35-14.42 years) alignment (p<0.001).

CONCLUSION

Postoperative varus malalignment appears to increase proportionally to the severity of preoperative varus deformity. Also, postoperative neutral alignment results in longer TKA survival time than residual varus alignment.

LEVEL OF EVIDENCE

III, Retrospective comparative cohort study.

Does the Preoperative Varus Deformity Influence the Survival of Postoperative Neutral-Aligned TKAs? An Analysis With a Minimum 5-Year Follow-Up

BACKGROUND

Postoperative neutral alignment may be an important factor for longevity of total knee arthroplasty (TKA). In knees with severe varus deformity, greater soft tissue release and bone resection were required to achieve neutral alignment. We investigated the relationship between the severity of preoperative varus deformity and longevity of neutral-aligned TKAs.

METHODS

Of the 723 knees in patients who underwent primary TKA for varus-type osteoarthritis between November 1998 and June 2009, 496 knees aligned neutrally (the postoperative mechanical hip-knee-ankle [HKA] axis angle ranged between -3° and 3°) and followed up for at least 5 years were included in the study. The mean follow-up period was 9.28 years. Patients were divided into 4 groups based on their preoperative HKAs: mild (0° < HKA ≤ 5°, n = 79), moderate (5° < HKA ≤ 10°, n = 204), severe (10° < HKA ≤ 15°, n = 149), and very severe (HKA > 15°, n = 64) groups. Failure was defined as need for revisional TKA for mechanical reason. Survival was analyzed by Kaplan-Meier method and log-rank test.

RESULTS

The overall failure rate was 2.02% (10 of 496 prostheses). The cumulative survival rates of neutral-aligned TKAs at 10 years were 97.4% (95% confidence interval [CI], 93.9%-100%), 99.0% (95% CI, 97.6%-100%), 97.8% (95% CI, 95.4%-100%), and 96.9% (95% CI, 92.6%-100%) in mild, moderate, severe, and very severe varus groups, respectively. There were no significant differences between group survival rates (P = .395).

CONCLUSION

The severity of preoperative varus deformity did not affect survival rates of neutral-aligned TKAs over 10 years.

Case-related factors affecting cutting errors of the proximal tibia in total knee arthroplasty assessed by computer navigation

PURPOSE

The aim of this study was to determine factors that contribute to bone cutting errors of conventional instrumentation for tibial resection in total knee arthroplasty (TKA) as assessed by an image-free navigation system. The hypothesis is that preoperative varus alignment is a significant contributory factor to tibial bone cutting errors.

METHODS

This was a prospective study of a consecutive series of 72 TKAs. The amount of the tibial first-cut errors with reference to the planned cutting plane in both coronal and sagittal planes was measured by an image-free computer navigation system. Multiple regression models were developed with the amount of tibial cutting error in the coronal and sagittal planes as dependent variables and sex, age, disease, height, body mass index, preoperative alignment, patellar height (Insall-Salvati ratio) and preoperative flexion angle as independent variables.

RESULTS

Multiple regression analysis showed that sex (male gender) (R = 0.25 p = 0.047) and preoperative varus alignment (R = 0.42, p = 0.001) were positively associated with varus tibial cutting errors in the coronal plane. In the sagittal plane, none of the independent variables was significant.

CONCLUSION

When performing TKA in varus deformity, careful confirmation of the bone cutting surface should be performed to avoid varus alignment. The results of this study suggest technical considerations that can help a surgeon achieve more accurate component placement.

LEVEL OF EVIDENCE

IV.

Selective medial soft tissue release combined with tibial reduction osteotomy in total knee arthroplasty

BACKGROUND

To obtain the correct coronal alignment and balancing in flexion and extension, we established a selective medial release technique and investigated the effectiveness and safety of the technique during primary total knee arthroplasty (TKA).

METHODS

Four hundred sixty-six primary TKAs with varus deformity were prospectively evaluated between June 2013 and June 2015. A knee joint position similar to Patrick's sign was used to release the medial structure. The medial release technique consisted of release of the capsule and the deep medial collateral ligament (dMCL) (step1), selective release of superficial medial collateral ligament (sMCL) or posterior oblique ligament (POL) (step 2), and selective tibial reduction osteotomy (step 3). Improvement of medial joint gap at each step and other clinical outcomes were evaluated.

RESULTS

Among the 466 knees, symmetrical gaps could be achieved by the limited release of the capsule and the dMCcL in 276 (59%) knees. One hundred fifty-two (33%) required additional sMCL release with 2-5 cm from the joint line distally or POL release. Thirty-eight (8%) necessitated an additional tibial reduction osteotomy. Anterior-medial release and 4-mm medial osteotomy contributed to more improvement of medial gap in flexion than in extension (each p < 0.01). Posteromedial release and posteromedial osteotomy contributed to more improvement in extension than in flexion (each p < 0.01). No specific complication related to our technique was identified.

CONCLUSION

The technique of the tibial reduction osteotomy combined with medial soft structure release using Patrick's sign is effective, safe, and minimally invasive to obtain balanced mediolateral and extension-flexion gaps in primary TKA.

A Comparison of the Medium-Term Results of Total Knee Arthroplasty Using Computer-Assisted and Conventional Techniques to Treat Patients With Extraarticular Femoral Deformities

BACKGROUND

We compared the medium-term results of total knee arthroplasty (TKA) performed using computer-assisted surgery (CAS) and conventional techniques in patients with extraarticular femoral deformities.

METHODS

The clinical and radiographic data of 40 knees (34 patients) with extraarticular femoral deformities that underwent consecutive CAS-TKA were compared with those of a control group that underwent conventional TKA (80 knees, 63 patients). No demographic data (in particular, the causes of the extraarticular deformities) differed between the two groups. The follow-up periods of the CAS and conventional TKA groups were 6.0 and 6.3 years, respectively.

RESULTS

In the CAS group, the average knee score increased from 48.2 to 89.6 at the final follow-up (P < .001); the average function score increased from 51.4 to 91.4. The range of motion averaged 105.5° preoperatively and 123.3° postoperatively. No significant differences were found in the knee score, function score, or range of motion after TKA between the 2 groups. The average preoperative mechanical axes were 18.1° varus in both groups. The average postoperative mechanical axis was 1.4° varus in the CAS group and 3.3° varus in the conventional TKA group (P = .001). The proportion of postoperatively aligned knees was 77.5% in the CAS group and 32.5% in the conventional group (P < .001).

CONCLUSION

Although the medium-term clinical results of TKA in extraarticular femoral deformities were satisfactory in both the CAS and conventional TKA groups, the former group exhibited more accurate postoperative coronal alignment. It was useful to employ navigation in patients with extraarticular deformities of various etiologies.

The Impact of Bone Deformity on Osteoarthritic Varus Knee Correctability

BACKGROUND

Bone deformities in the varus osteoarthritic knee may influence soft-tissue balancing and therefore knee correctability. The hypothesis of the present study was that the grade of coronal plane knee deformity may influence directly knee correctability along the entire range of motion from 0° to 90°. Tibial and femoral epiphyseal bone deformities were also analyzed to determine which kind had the greater impact on knee correctability.

METHODS

A coronal plane deformity radiographic assessment and an intraoperative correctability assessment using computer-assisted surgery were performed on 118 varus osteoarthritic knees undergoing total knee arthroplasty. Knees were divided into groups taking into account the kind of bone deformity (tibial, femoral, and combined).

RESULTS

A significant inverse correlation was found between coronal plane deformity and knee correctability at every 10 degrees of flexion. Correlation was strong at 0° and progressively got weaker at further flexion angles. According to literature, knees with a varus deformity >10° were rarely correctable in full extension, but often correctable in flexion, whereas knees with varus deformity >15° showed to be almost never correctable. Combined deformity group had a significantly lower rate of correctability along the entire range of motion.

CONCLUSION

The severity of varus knee malalignment always influenced knee correctability with the knee in full extension, in further flexion (20°-60°), correctability was mildly affected. Isolated tibial epiphyseal deformity and combined epiphyseal deformity have the greatest impact on knee correctability.

Preoperative Measurement of Tibial Resection in Total Knee Arthroplasty Improves Accuracy of Postoperative Limb Alignment Restoration

Background:

Accuracy of implant placement in total knee arthroplasty (TKA) is crucial. Traditional extramedullary alignment instruments are fairly effective for achieving the desired mean tibial component coronal alignment. We modified the traditional tibial plateau resection technique and evaluated its effect on alignment restoration.

Methods:

Two hundred and eighty-two primary TKAs in our hospital between January 2013 and December 2014 were enrolled in this retrospective study. Group A consisted of 128 primary TKAs performed by one senior surgeon. Preoperative measurement of the tibial resection was conducted on radiographs, and the measured thicknesses of the lateral and medial plateau resection were used to place the tibial alignment guide. Group B consisted of 154 primary TKAs performed by the other senior surgeon, using a traditional tibial plateau resection technique. In all patients, an extramedullary guide was used for tibial resection, and preoperative and postoperative full-leg standing radiographs were used to assess the hip-knee-ankle angle (HKA), femoral component alignment angle (FA), and tibial component alignment angle (TA). A deviation ≥3° was considered unsatisfactory. Data were analyzed by unpaired Student's t-test.

Results:

The mean postoperative HKA and TA angles were significantly different between Groups A and B (178.2 ± 3.2° vs. 177.0 ± 3.0°, t = 2.54, P = 0.01; 89.3 ± 1.8° vs. 88.3 ± 2.0°, t = 3.75, P = 0.00, respectively). The mean postoperative FA was 88.9 ± 2.5° in Group A and 88.9 ± 2.6° in Group B, and no significant difference was detected (t = 0.10, P = 0.92). There were 90 (70.3%) limbs with restoration of the mechanical axis to within 3° of neutral alignment and 38 (29.7%) outliers (>3° deviation) in Group A, whereas there were 89 (57.8%) limbs with restoration of the mechanical axis to within 3° of neutral alignment and 65 (42.2%) outliers (>3° deviation) in Group B. The severity of the preoperative alignment deformity was a strong predictor for postoperative alignment.

Conclusions:

Using conventional surgical instruments, preoperative measurement of resection thickness of the tibial plateau on radiographs could improve the accuracy of conventional surgical techniques.

Risk factors for tibial implant malpositioning in total knee arthrosplasty-consecutive series of one thousand, four hundred and seventeen cases

PURPOSE

Total knee arthroplasty (TKA) malalignment may result in pain and limited range of motion. The present study assessed the influence of different surgeon's and patient's related factors on the post-operative tibial tray coronal alignment.

METHODS

The charts and the x-rays of a continuous prospective series of 1417 TKAs operated upon between 1987 and 2015 were retrospectively reviewed. The long-leg AP views were performed at two months post-op and the tibial mechanical angle of the tibial tray was measured. Three groups were defined: varus (≤87° n = 167), valgus (≥93° n = 55) and well alignment (88° to 92° n = 1195). The influence of several pre-operative and peri-operative factors was investigated: surgeon handedness and experience (junior or senior), previous tibial osteotomies, Ahlbäck stage of osteoarthritits, pre-operative alignment, height and weight, age at surgery, approach (medial, lateral or tibial tubercle osteotomy), generation of implants, tray fixation, size of the tray and stem lenght. Univariate then multivariate analysis were performed to find out any correlation.

RESULTS

Multivariate analysis showed a strong correlation between varus alignment of the tibial tray and pre-operative varus of the lower limb (p = 0.037), increased BMI (p = 0.016) and operated side opposite to the dominant surgeon's arm (p = 0.006). In a similar way a strong correlation was found between valgus alignment and pre-operative valgus of the limb (p = 0.026).

CONCLUSION

Poor alignment of the tibial tray after TKA was associated with pre-operative malalignment of the lower limb, increased BMI and an index knee which was opposite to surgeon's dominant arm.

Shorter survival rate in varus-aligned knees after total knee arthroplasty

PURPOSE

One long-held tenet of total knee arthroplasty (TKA) is that post-operative neutral limb alignment promotes implant durability. Recently, the concept of generic safe zone (0° ± 3°) has been challenged. This meta-analysis aimed to evaluate whether neutral alignment was superior to malalignment in long-term survival of TKAs.

METHODS

The MEDLINE, Embase, Cochrane Library, China National Knowledge Infrastructure, Wan Fang Chinese Periodical, Google and reference lists of all the included studies were searched. Of the 1512 studies initially identified, ten met the eligibility criteria, including eight case-control studies and two cohort trials. Relative risks of implant failure were compared between post-operative neutrally aligned and malaligned knees.

RESULTS

Post-operative malalignment showed higher failure rate of knee implants compared with neutral alignment (95 % CI 1.00-1.88, P = 0.05). Failure rate in knees with varus alignment was significantly higher than with neutral alignment (95 % CI 1.07-2.55, P = 0.02). There was no significant difference in the likelihood of implant failure between knees with valgus and neutral alignment (95 % CI 0.78-2.41, n.s.). No significant difference of failure rate was noted between neutral alignment and malalignment for fixed-bearing prothesis (95 % CI 0.94-1.95, n.s.) or rotating-platform prothesis (95 % CI 0.75-2.73, n.s.). There was no significant difference of failure rate between knees with neutral alignment and malalignment for studies with a mean follow-up of more than 10 years (95 % CI 0.81-2.01, n.s.) or studies using long-leg weight-bearing radiographs (95 % CI 0.79-1.79, n.s.).

CONCLUSIONS

Post-operative varus alignment results in shorter survival rate after TKA. Not only neutral limb alignment but also the valgus alignment promotes implant durability. Neutral or valgus alignment rather than varus alignment is essential to achieve long-term survival of TKAs and patient satisfaction.

LEVEL OF EVIDENCE

III.

Alignment in knee flexion position during navigation-assisted total knee arthroplasty

PURPOSE

The aim of this study was to demonstrate the lower limb alignment in knee flexion position after navigation-assisted total knee arthroplasty using the gap technique and to identify the correlative factors.

METHODS

One hundred and twenty consecutive osteoarthritic knees (120 patients) were prospectively enrolled for intraoperative data collection. All TKA surgeries were performed using the navigation system (OrthoPilot™, version 4.0; B. Braun Aesculap, Tuttlingen, Germany). Before and after final prosthesis implantation, the lower limb navigation alignment in both knee extension (0°) and knee flexion (90°) position was recorded. The knee flexion alignment was divided into three groups: varus, neutral and valgus alignment. To determine the factors of the alignment in knee flexion position, preoperative demographics, radiologic and intraoperative data were obtained. Pearson's correlation (r) analysis was performed to find the correlation. The Knee Society Score and Western Ontario and McMaster Universities Osteoarthritis Index were compared between groups.

RESULTS

Although all postoperative extension alignment was within neutral position (between -2° and +2°), postoperative knee flexion alignment was divided into three groups: varus (≤-3°), 24 cases (20 %); neutral (between -2° and +2°), 85 cases (70.8 %) and valgus (≥+3°) alignment, 11 cases (9.2 %). There were a good correlation of alignment in knee flexion position with the rotation of femoral component relative to posterior condylar axis (r = -0.502, p = 0.000) and weak correlations with posterior femoral cut thickness (lateral condyle) (r = 0.207, p = 0.026), medial flexion (90°) gap after femoral component rotation adjustment (r = 0.276, p = 0.003). Other variables did not show correlations. There were no statistical clinical differences between varus, neutral and valgus knee flexion alignment groups.

CONCLUSION

About 30 % of the cases showed malalignment of more than 3° in knee flexion position although with neutral alignment in extension position. The knee flexion alignment had a good correlation with the rotation of femoral component relative to posterior condylar axis. Neutral alignment in knee flexion position may be adjusted by femoral component rotation especially by the use of navigation system.

LEVEL OF EVIDENCE

IV.

Does patella position influence ligament balancing in total knee arthroplasty?

PURPOSE

In vivo comparative gap measurements were performed in three different patella positions (reduced, subluxated and everted) using offset-type-force-controlled-spreader-system.

METHODS

Prospectively, 50 knees were operated by total knee arthroplasty using a navigation-assisted gap-balancing technique. The offset-type-force-controlled-spreader-system was used for gap measurements. This commercially available instrument allows controllable tension in patella-reduced position. The mediolateral gaps of knee extension (0°) and flexion (90°) angle were recorded in three different patella positions; reduced, subluxated and everted. Any gap differences of more than 3 mm were considered as a meaningful difference. Correlation between the difference with the demographic data, preoperative radiologic alignment and intraoperative data was analysed. For statistical analysis, ANOVA and Pearson's correlation test were used.

RESULTS

The gaps in patella eversion demonstrated smaller gaps both in knee extension and flexion position compared to the gaps of patella reduction position. The amount of decreased gaps was more definite in knee flexion position. Statistically significant difference was observed for the lateral gap of patella eversion compared to gap of patella reduction in knee flexion position (p < 0.05). There were notable cases of variability in knee flexion position. Significant portion of 12 (24 %) knees of patella subluxation and 33 (66 %) knees of patella evertion demonstrated either increased or decreased gaps in knee flexion position compared to the gaps of patella reduction position.

CONCLUSION

The gaps in patella eversion demonstrated smaller gaps both in knee extension and flexion position compared to the gaps of patella reduction position. The amount of decreased gaps was more definite in knee flexion position. Therefore, the intraoperative patellar positioning has influence on the measurement of the joint gap. Keeping the patella in reduced position is important during gap balancing.

LEVEL OF EVIDENCE

I.