Magnetic resonance imaging of the knee after medial unicompartmental arthroplasty

Objective

We evaluated safety and potential diagnostic value of magnetic resonance (MR) imaging of the knee treated with medial unicompartmental arthroplasty (MUA).

Methods

The treated knee of 8 patients who underwent MUA was studied with four different 1.5-T MR sequences. Two radiologists independently evaluated eleven anatomical items using a score from 0 (not assessable) to 3 (completely assessable). The sum of the scores for each sequence was divided by the potential maximal sum, obtaining a percentage visibility index (PVI) for each item.

Results

No adverse effect was reported during or within 30 min after the exam. Posterior cruciate ligament was unseen in all patients by both observers. The following PVIs were reported for the remaining ten items: femoral–patellar relationship 83–100%; femoral–patellar cartilage 92–100%; Hoffa's fat pad 75–92%; patellar ligament 79–100%; lateral meniscus 100%; femoral–tibial lateral joint 100%; lateral collateral ligament 96–100%; anterior cruciate ligament 54–83%; femoral–tibial lateral cartilages 92–100%; posterolateral corner 100%. Agreement between readers was found in 331/352 (94%) evaluations (k = 0.74–0.78).

Conclusions

MR imaging after MUA offers a safe and reproducible evaluation of residual knee anatomy except for cruciate ligament, and can be used to follow-up these patients.     

High rate of implant loosening for uncemented resurfacing-type medial unicompartmental knee arthroplasty

Purpose

To determine the medium-term implant survivorship, the clinical results and the failure mechanisms of a novel unicompartmental arthroplasty for uncemented resurfacing of the medial tibio-femoral compartment.

Methods

Seventy-six consecutive patients were prospectively evaluated with a mean final follow-up of 6 years (SD 5.3 months). In 44 patients, the diagnosis was osteoarthritis, and in 32 patients, it was avascular necrosis of the medial femoral condyle. The Hospital for Special Surgery Score was used for objective clinical evaluation, and a self-administered visual analogue scale was used to quantify residual pain at each observation point. Implant survivorship was determined assuming revision for any reason as endpoint.

Results

Nineteen patients were revised (6 with osteoarthritis and 13 with avascular necrosis of the medial femoral condyle). The mean interval time from index surgery to revision was 11.2 months (SD 4.66 months). Implant survivorship was higher in patients with osteoarthritis with respect to those with avascular necrosis of the medial femoral condyle (p = 0.018). Aseptic loosening was the most frequent failure mechanism. Femoral component loosening was reported in five patients and tibial component loosening was reported in other six patients. Assuming revision for any reason as endpoint, an implant survivorship of 74.3 % at 6-year follow-up was determined. In the remaining 57 patients, satisfactory clinical results were obtained. Hospital for Special Surgery Score and visual analogue scale for residual pain showed significant improvements (p < 0.03 and p < 0.045, respectively).

Conclusions

At the present time, the standard cemented implants and the conventional designs for unicompartmental knee replacement still represent the optimal solution. The authors do not recommend the widespread use of this technique.

Level of evidence

IV.

     

The minimally invasive far medial subvastus approach for total knee arthroplasty in valgus knees

Purpose

The lateral approach in the valgus knee asks for a lot of soft tissue releases during the arthrotomy. The hypothesis of this study was that the far medial subvastus approach could be used in valgus knees and would guarantee both functional and radiological good to excellent results.

Methods

This is a retrospective study on 78 patients (84 knees) undergoing primary total knee arthroplasty (TKA) for type I or II fixed valgus knees. The mean (SD) preoperative mechanical alignment was 187° (4°) HKA angle. Functional recovery, pain, tourniquet times, necessary soft tissue releases as well as radiological alignment were measured.

Results

The Knee Score improved significantly from 45 (10) to 90 (10) (P < 0.05) and the function score improved as well from 35 (20) to 85 (10) (P < 0.05). Flexion improved from 110° (10°) to 137° (8°). Hospital stay was 4 (1.2) days. Alignment was corrected to 181° (1.5°) HKA angle with a postoperative joint line shift of +2.8 (3.2) mm. No clinical instability, as evaluated by the senior author, or osteolytic lines was observed at minimal one-year radiological follow-up.

Conclusion

The far medial subvastus approach is an excellent approach to perform Krackow type I and II TKA with primary PS implants.

Level of evidence

IV.     

No influence of coronal laxity and alignment on lift-off after well-balanced and aligned total knee arthroplasty

Purpose

In vivo fluoroscopic analyses have revealed the kinematics after total knee arthroplasty (TKA), including femoral condylar lift-off. This study asked whether differences in static varus–valgus laxity or coronal limb alignment after TKA affect lift-off under weight-bearing conditions. It was hypothesised that there is a correlation between coronal laxity or alignment and lift-off during walking.

Methods

The current study analysed nineteen subjects undergoing cruciate-retaining TKA performed by the measured resection technique. The varus–valgus laxity at knee extension was measured using a 150 N stress radiograph. The mechanical axis was measured using a full-standing radiograph. Continuous radiological images were taken while the subject walked on a treadmill, and the images during single-leg stance were analysed to determine the lift-off using a 3D-to-2D image-to-model registration technique.

Results

The average angle in varus/valgus stress was 6.8 ± 1.8°/6.6 ± 2.1°. No statistically significant differences were observed between the varus and valgus laxity. The average amount of lift-off was 0.7 ± 0.4 mm. The static varus–valgus laxity (n. s.) or the differences in the laxities (n. s.) on the stress radiograph did not influence lift-off. The weight-bearing ratio was achieved within the middle third of the knee in 90 % of subjects. Two outliers with valgus alignment (68 ± 1 %) demonstrated no significant difference in lift-off in comparison with the majority of the subjects (46 ± 9 %).

Conclusion

The static coronal laxity and alignment did not influence the lift-off under dynamic weight-bearing conditions after well-balanced and aligned cruciate-retaining TKA. Measured resection technique can produce sufficient coronal stability and alignment without significant lift-off during walking.

Level of evidence

IV.     

Joint line reconstruction in medial unicompartmental knee arthroplasty: development and validation of a measurement method

Purpose

Preservation of the joint line in total knee arthroplasty (TKA) has shown to be an important factor for the long-term outcome, especially in revision TKA. For unicompartmental knee arthroplasty (UKA), the role of the joint line has neither been investigated nor is it consciously respected during implantation. Thus, the aim was to establish and validate a standardised measurement method to determine the joint line in UKA.

Methods

As there is no established method to evaluate changes in the joint line radiologically, we introduced two methods and correlated them. The methods were first validated in a cadaver model by a controlled rotational study. Then, the joint line of 29 patients with an UKA (Oxford, Biomet, Bridgend, UK) was determined on pre- and post-operative radiographs. Both methods were tested by intra- and inter-rater reliability.

Results

Both methods showed a good intra- and inter-rater reliability. Furthermore, there was only little bias in agreement between both methods and raters. Measurements of the 29 UKA patients revealed that the joint line was more distally by a mean of 4.4 ± 1.2 mm after surgery.

Conclusions

The study provides for the first time a reliable and standardised measurement tool to determine the changes in the joint line after implantation of an UKA. The instrument should be used in further studies to evaluate the impact of the joint line on the long-term outcome, the load in the two non-replaced knee compartments and on the ligaments.

Level of evidence

Diagnostic study without a universally applied ‘gold’ standard, Level III.     

The WOMAC score can be reliably used to classify patient satisfaction after total knee arthroplasty

Purpose

The primary aim of this study was to define a classification in the WOMAC score after total knee arthroplasty (TKA) according to patient satisfaction. The secondary aims were to describe patient demographics for each level of satisfaction.

Methods

A retrospective cohort consisting of 2589 patients undergoing a primary TKA were identified from an established arthroplasty database. Patient demographics, Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), and short form (SF) 12 scores were collected pre-operatively and 1 year post-operatively. In addition, patient satisfaction was assessed at 1 year with four responses: very satisfied, satisfied, dissatisfied or very dissatisfied. Receiver operating characteristic (ROC) curves were used to identify values in the components and total WOMAC scores that were predictive of each level of satisfaction, which were used to define the categories of excellent, good, fair and poor.

Results

At 1 year, there were 1740 (67.5%) very satisfied, 572 (22.2%) satisfied, 190 (7.4%) dissatisfied and 76 (2.9%) very dissatisfied patients. ROC curve analysis identified excellent, good, fair and poor categories for the pain (>?78, 59–78, 44–58, <?44), function (>?72, 54–72, 41–53, <?41), stiffness (>?69, 56–69, 43–55, <?43) and total (>?75, 56–75, 43–55, <?43) WOMAC scores, respectively. Patients with lung disease, diabetes, gastric ulcer, kidney disease, liver disease, depression, back pain, with worse pre-operative functional scores (WOMAC and SF-12) and those with less of an improvement in the scores, had a significantly lower level of satisfaction.

Conclusion

This study has defined a post-operative classification of excellent, good, fair and poor for the components and total WOMAC scores after TKA. The predictors of level of satisfaction should be recognised in clinical practice and patients at risk of a lower level of satisfaction should be made aware in the pre-operative consent process.

Level of evidence

III.

     

Tibial component alignment and risk of loosening in unicompartmental knee arthroplasty: a radiographic and radiostereometric study

Purpose

Unicompartmental knee arthroplasty (UKA) has shown a higher rate of revision compared with total knee arthroplasty. The success of UKA depends on prosthesis component alignment, fixation and soft tissue integrity. The tibial cut is the crucial surgical step. The hypothesis of the present study is that tibial component malalignment is correlated with its risk of loosening in UKA.

Methods

This study was performed in twenty-three patients undergoing primary cemented unicompartmental knee arthroplasties. Translations and rotations of the tibial component and the maximum total point motion (MTPM) were measured using radiostereometric analysis at 3, 6, 12 and 24 months. Standard radiological evaluations were also performed immediately before and after surgery. Varus/valgus and posterior slope of the tibial component and tibial–femoral axes were correlated with radiostereometric micro-motion. A survival analysis was also performed at an average of 5.9 years by contacting patients by phone.

Results

Varus alignment of the tibial component was significantly correlated with MTPM, anterior tibial sinking, varus rotation and anterior and medial translations from radiostereometry. The posterior slope of the tibial component was correlated with external rotation. The survival rate at an average of 5.9 years was 89 %. The two patients who underwent revision presented a tibial component varus angle of 10° for both.

Conclusions

There is correlation between varus orientation of the tibial component and MTPM from radiostereometry in unicompartmental knee arthroplasties. Particularly, a misalignment in varus larger than 5° could lead to risk of loosening the tibial component.

Level of evidence

Prognostic studies—retrospective study, Level II.     

Flexion and extension laxity after medial, mobile-bearing unicompartmental knee arthroplasty: a comparison between a spacer- and a tension-guided technique

Purpose

In a mobile-bearing unicompartmental knee arthroplasty (UKA), stability is of utmost importance to promote knee function and to prevent dislocation of the insert. Gap balancing can be guided by the use of spacers or a tensioner. The goal of this study is to compare laxity of a tension-guided implantation technique versus a spacer-guided technique for medial UKA with a mobile bearing. Also clinical function was compared between the groups.

Methods

The tension-guided UKA system (BalanSys?, Mathys Ltd, Bettlach, Switzerland) was compared with a retrospective group with a spacer-guided system (Oxford, Biomet Ltd, Bridgend, UK). A total of 30 tension-guided medial UKAs were implanted and compared with 35 spacer-guided medial prostheses. In both groups, valgus laxity was measured at least 4 months postoperatively in extension and 70° flexion using stress radiographs. Knee Society Scores (KSS) were obtained at the 6-month follow-up.

Results

Valgus laxity in flexion was significantly higher in the tension-guided group compared with the spacer-guided group: 3.9° (SD 1.8°) versus 2.4° (SD 1.2°), respectively, P < 0.001). In extension, valgus laxity was significantly different: 1.8° (SD 1.0°) in the tension-guided group compared with 2.7° (SD 0.9°) in the spacer-guided group (P < 0.001). There was no significant difference between the KSS for the two groups (n.s.).

Conclusions

The tensor-guided system resulted in significantly more valgus laxity in flexion compared with the spacer-guided system. However, in extension, the situation was reversed: the tension-guided system resulted in less valgus laxity than the spacer-guided system. Clinically, there were no differences between the groups. The valgus laxity found with the spacer-guided system better approximates the valgus laxity values of the healthy elderly.

Level of evidence

Lower quality prospective cohort study (<80 % follow-up, patients enrolled at different time points in disease), Level II.     

Comparative study of the use of computer assisted navigation system for axial correction in medial unicompartmental knee arthroplasty

The aim of our study was to compare the use of the Orthopilot Navigation system with conventional non-navigation technique for medial UKA with respect to the intraoperative mechanical limb alignment measurements and correlation with the postoperative radiological measurements. The postoperative mechanical limb alignment axes of 51 consecutive medial unicompartmental knee arthroplasty performed by a single surgeon over a 12-month period were measured. The cases were randomly assigned to two groups of which 21 cases were performed using conventional non-navigation based technique and 30 cases were performed using the Orthopilot Navigation System. Computed tomography (CT) scanogram was performed for all cases within the same hospitalization stay to assess the postoperative mechanical limb alignment. Our results showed that the non-navigated group had a more neutral mechanical axis with a narrower range compared to the navigation assisted group. The difference in the mean mechanical axis between the two groups was statistically not significant. There was poor correlation between the intraoperative navigation system measurements and the postoperative radiological measurements. In conclusion, the use of computer navigation in UKA is not as well validated as compared to TKA. We did not demonstrate any improvement in postoperative axial limb alignment measurement in using a computer navigation system compared to conventional non-navigation technique.     

MR imaging of the knee in patients with medial unicompartmental arthroplasty: comparison among sequences at 1.5 T

Purpose

This study was done to test a series of magnetic resonance (MR) imaging sequences of the knee after medial unicompartmental arthroplasty.

Materials and methods

Four patients who had undergone Oxford III medial unicompartmental arthroplasty underwent 1.5-T MR imaging of the operated knee using coronal sequences: T1-weighted spin-echo (SE), T1-weighted turbo SE (TSE), proton-density (PD)- and T2-weighted TSE, T1-weighted gradient echo (GE), short-tau inversion recovery (STIR), multi echo data image combination (MEDIC), T2*-weighted GE, volumetric interpolated breath-hold examination (VIBE), and dual-echo steady state (DESS). For each sequence, we evaluated the visibility of the anatomical structures of the central pivot, lateral compartment, and anterior compartment using a semiquantitative score (0=total masking; 1=insufficient visibility; 2=sufficient visibility; 3=optimal visibility). The sum of the scores given to each sequence was divided by the maximal sum, obtaining a percentage visibility index. Friedman and sign tests were used for statistical analysis.

Results

MR examination time was 30–32 min. No patients reported pain, heat or other local discomfort. The visibility index ranged between 83% and 89% for the first four sequences without significant differences among them, 58% for STIR and 11%–36% for the last five sequences. Significant differences were found between each of the four first sequences and the remaining sequences (p<0.004) and between STIR and the last five sequences (p<0.008).

Conclusions

MR imaging of the knee after medial unicompartmental arthroplasty was not associated with adverse events. An imaging protocol including SE, TSE and STIR sequences could be used to study the knee with unicompartmental arthroplasty.     

Total knee arthroplasty in severe valgus knee deformity: comparison of a standard medial parapatellar approach combined with tibial tubercle osteotomy

Purpose  

Primary TKA in valgus knees with a deformity of more than ten degrees may prove challenging, since bone and soft tissue abnormalities make accurate axis restoration, component orientation and joint stability attainment a difficult task. The purpose of this study was to determine which approach is optimal in these patients, by comparing the standard medial parapatellar approach to a lateral parapatellar combined with a tibial tubercle osteotomy (TTO).     

拉网式松解技术在人工膝关节置换术治疗重度外翻膝中临床应用

目的探讨拉网式松解技术在人工膝关节置换术治疗外翻膝中的安全性及疗效。方法回顾性分析2014年10月至2015年5月北京积水潭医院矫形骨科采用人工膝关节置换术治疗的膝关节外翻畸形患者20例(23膝)的临床资料。所有患者均进行膝关节置换手术治疗,在术后1、3、6个月及1年进行随访,拍摄膝关节负重位X线影像、下肢全长X线影像,观察有无腓总神经损伤的症状,并进行美国膝关节协会(KSS)评分。结果本组20例患者,18例使用后稳定型假体,2例使用限制性假体。全部患者膝关节置换术中行外侧软组织的拉网式松解,松解深度≤5 mm。平均手术时间(79±12)min,平均出血量(150±30)ml,术中及术后无输异体血,术后伤口平均引流量(190±70)ml。术后第1天患者可以扶拐杖下地活动。术后和随访过程中均未发现腓总神经损伤的并发症。KSS评分从术前的(39±10)分提高至术后的(91±6)分,术前、术后KSS评分比较,差异有统计学意义(P<0.05)。结论拉网式松解技术是人工膝关节置换术治疗膝关节外翻畸形的有效方法,松解深度≤5 mm安全、有效,可以得到满意的软组织平衡。     

The contralateral knee may not be a valid control for biomechanical outcomes after unilateral total knee arthroplasty

BackgroundAlthough unilateral symptoms and unilateral total knee arthroplasty (TKA) are common, many patients have bilateral radiographic osteoarthritis (OA). Because the contralateral (non-operated) limb is often used as a comparison for clinical and biomechanical outcomes, it is important to know if the presence of OA influences movement patterns in either limb.Research questionThe purpose of this study was to compare bilateral sagittal plane biomechanics between subjects with and without contralateral knee OA after unilateral TKA.MethodsFifty-three subjects who underwent unilateral TKA underwent three-dimensional gait analysis 6–24 months after surgery participated in this cross-sectional study. Kellgren-Lawrence (KL) OA severity in the contralateral limb was measured, and subjects were classified into either a non-OA (KL 0 or 1) or OA (KL 2–4) group. Mixed-model ANOVA tests with factors of group and limb were used to compare biomechanical measures. In the presence of a significant interaction effect, post-hoc comparisons were performed.ResultsThe OA group had more knee flexion at initial contact, less knee flexion and extension excursions, and less knee extension in the contralateral limb compared to the non-OA group. The non-OA group had significant differences between limbs, with more knee flexion at initial contact, less knee joint excursion, and less peak knee extension on the operated limb compared to the contralateral limb, whereas there were no limb differences for the OA group. Kinetic variables were not different in the ANOVA models.Significance and interpretationSubjects with contralateral knee OA have more symmetrical gait, although they adopt a more abnormal and stiff-legged gait pattern bilaterally. Researchers and clinicians should consider radiographic disease severity, not just symptoms, in the contralateral limb when identifying appropriate subject samples for unilateral biomechanical studies. Symmetrical movement patterns between limbs after surgery should not be the sole factor upon which movement recovery is based.     

The medial border of the tibial tuberosity as an auxiliary tool for tibial component rotational alignment during total knee arthroplasty (TKA)

Purpose

The objective of this study was to quantify the amount of ensuing internal rotation of the tibial component when positioned along the medial border of the tibial tubercle, thus establishing a reproducible intraoperative reference for tibial component rotational alignment during total knee arthroplasty (TKA).

Methods

The angle formed from the tibial geometric centre to the intersection of both lines from the middle of the tibial tuberosity and its medial border was measured in 50 patients. The geometric centre was determined on an axial CT slice at 10 mm below the lateral tibial plateau and transposed to a slice at the level of the most prominent part of the tibial tuberosity. Similar measurements were taken in 25 patients after TKA, in order to simulate the intraoperative appearance of the tibia after making its proximal resection.

Results

This angle was found to be similar (n.s.) in normal and post-TKA tibiae [median 20.4° (range 15°–24°) vs. 20.7° (range 16°–25°), respectively]. In 89.3 % of the patients, the angle ranged from 17° to 24°. No statistical difference (p n.s.) was found between women and men in both normal [median ?20.7° (range 16°–25°) vs. 19.9° (range 15°–24°)] and post-TKA tibiae [median 21.4° (range 19°–24°) vs. 20° (range 16°–25°)].

Conclusion

This study found that in 90 % of the patients, the medial border of the tibial tuberosity is internally rotated 17°–24° in relation to the line connecting the middle of the tuberosity to the tibial geometric centre. Since this anatomical landmark may be more easily identifiable intraoperatively than the commonly used “medial 1/3”, it can provide a better quantitative reference point and help surgeons achieve a more accurate tibial implant rotational position.

Level of evidence

Cohort and case control studies, Level III.

     

The accuracy of the extramedullary and intramedullary femoral alignment system in total knee arthroplasty for varus osteoarthritic knee

Purpose

The intramedullary (IM) femoral alignment system does not alway guarantee accuracy of the component position in the total knee arthroplasty (TKA). In some cases, the extramedullary (EM) femoral alignment system in total knee arthroplasty (TKA) is a useful alternative surgical option to adjust femoral component alignment. In the EM technique, accuracy of the femoral head center location is mandatory. The purpose of this prospective randomized study was to compare the alignment after TKA using two different femoral alignment systems.

Methods

From January 2009 to December 2009, 91 patients (106 knees) with osteoarthritis underwent TKA. The IM femoral alignment system was used in 50 TKAs, and the EM system was used in 56 TKAs. We measured the coronal, sagittal alignment of the femoral component, and overall alignment from full-length standing. Anteroposterior radiographs were taken 1 year after surgery.

Results

The overall limb alignment was 0.2° ± 1.9° varus in the EM group and 1.1° ± 1.9° valgus in the IM group (p = 0.001). The coronal alignment of the femoral component was 90.0° ± 1.1° in the EM group and 90.3° ± 1.2° in the IM group, not statistically different (n.s.). The sagittal alignment of the femoral component was 2.3° ± 1.7° in the EM group and 2.5° ± 1.0° in the IM group (n.s.). Clinically acceptable overall limb alignment was achieved in 91.1 % of EM group and 84.0 % of IM group (n.s.).

Conclusion

The present study suggests that by applying our EM technique that uses a newly designed mechanical axis marker system, the alignment of the femoral component and overall limb alignment is reliable and at least as accurate as the standard IM technique.

Level of evidence

I.     

The "moving valgus stress test" for medial collateral ligament tears of the elbow

BACKGROUND: The diagnosis of a painful partial tear of the medial collateral ligament in overhead-throwing athletes is challenging, even for experienced elbow surgeons and despite the use of sophisticated imaging techniques. HYPOTHESIS: The "moving valgus stress test" is an accurate physical examination technique for diagnosis of medial collateral ligament attenuation in the elbow. STUDY DESIGN: Cohort study (diagnosis); Level of evidence, 2. METHODS: Twenty-one patients underwent surgical intervention for medial elbow pain due to medial collateral ligament insufficiency or other abnormality of chronic valgus overload, and they were assessed preoperatively with an examination called the moving valgus stress test. To perform the moving valgus stress test, the examiner applies and maintains a constant moderate valgus torque to the fully flexed elbow and then quickly extends the elbow. The test is positive if the medial elbow pain is reproduced at the medial collateral ligament and is at maximum between 120 degrees and 70 degrees. RESULTS: The moving valgus stress test was highly sensitive (100%, 17 of 17 patients) and specific (75%, 3 of 4 patients) when compared to assessment of the medial collateral ligament by surgical exploration or arthroscopic valgus stress testing. The mean shear range (ie, the arc within which pain was produced with the moving valgus stress test) was 120 degrees to 70 degrees. The mean angle at which pain was at a maximum was 90 degrees of elbow flexion. CONCLUSIONS: The moving valgus stress test is an accurate physical examination technique that, when performed and interpreted correctly, is highly sensitive for medial elbow pain arising from the medial collateral ligament.     

Revision total knee arthroplasty: a comparison of postoperative leg alignment after computer-assisted implantation versus the conventional technique

Accurate reconstruction of leg alignment is one important factor for long-term survival in total knee arthroplasty (TKA). Recent developments in computer-assisted surgery focused on systems improving TKA. The aim of the study is to compare the results of computer-assisted revision TKA with the conventional technique. We hypothize that a significantly better leg alignment and component orientation is achieved when using a navigation system for revision TKA. In a prospective study, two groups of 25 revision TKAs each were operated on using either a CT-free navigation system or the classical surgeon-controlled technique. The postoperative leg alignment was analysed on long-leg coronal and lateral X-rays. The mechanical limb axis was significantly better in the navigation-based group. Twenty-three patients (92%) in the computer-assisted group had a postoperative leg axis between 3° varus/valgus deviation, while 19 patients (76%) in the conventional group had a comparable result (p<0.05). Further, significant differences were seen for the coronal orientation of the femoral component. Computer-assisted revision TKA leads to a superior restoration of leg alignment compared with the conventional technique. Particularly the real-time presentation of the actual leg axis and the flexion and extension gaps is useful in revision TKA. Potential benefits in long-term outcome and functional improvement require additional investigation.