Patellar resection during total knee arthroplasty: effect on bone strain and fracture risk

Patellae in small knees, or after severe patellar erosion, may be vulnerable to fracture after resection during arthroplasty. The patellar remnant may be thin, while the polyethylene component has a standard thickness. Anterior patellar bone strain was measured in cadaver knees loaded via the quadriceps, from 0 to 90° flexion, with the patella intact, and after resections to 16, 13 and 11 mm thick and replacement by an 8-mm-thick polyethylene component. Strain increased significantly with knee flexion with constant 500 N quadriceps tension. Resection caused significant changes from intact values in knee flexion and extension and no significant effect at 30° flexion. In flexion, bending caused the anterior surface to become more convex, with high tensile bone strains. In extension, resection caused negative anterior strains, representing bending in the opposite direction, with large tensile strains on the cut posterior surface. For normal activities, such as rising from a chair (1.8 kN quadriceps tension) the patella appears safe against fracture with a minimal resection to 16 mm thick. An eroded patella resected to 11 mm thickness may be at risk of fracture with that loading.     

Increased patellofemoral pressure after TKA: an in vitro study

Purpose

Considering the discrepant results of the recent biomechanical studies, the purpose of this study was to simulate dynamic muscle-loaded knee flexion with a large number of specimens and to analyse the influence of total knee arthroplasty (TKA) without and with patellar resurfacing on the patellofemoral pressure distribution.

Methods

In 22 cadaver knee specimens, dynamic muscle-loaded knee flexion (15°–90°) was simulated with a specially developed knee simulator applying variable muscle forces on the quadriceps muscles to maintain a constant ankle force. Patellofemoral pressures were measured with flexible, pressure-sensitive sensor foils (TEKSCAN) and patellofemoral offset with an ultrasound motion-tracking system (ZEBRIS). Measurements were taken on the native knee, after total knee arthroplasty and after patellar resurfacing. Correct positioning of the patellar implant was examined radiologically.

Results

The maximal patellofemoral peak pressure partly increased from the native knee to the knee with TKA with intact patella (35°–90°, p < 0.012) and highly increased (twofold to threefold) after patellar resurfacing (20°–90°, p < 0.001). Concurrently, the patellofemoral contact area decreased and changed from a wide area distribution in the native knee, to a punctate area after TKA with intact patella and a line-shaped area after patellar resurfacing. Patellar resurfacing led to no increase in patellar thickness and patellofemoral offset.

Conclusions

Despite correct implantation of the patellar implants and largely unchanged patellofemoral offset, a highly significant increase in pressure after patellar resurfacing was measured. Therefore, from a biomechanical point of view, the preservation of the native patella seems reasonable if there is no higher grade patellar cartilage damage.     

Asymmetrical total knee arthroplasty does not improve patella tracking: a study without patella resurfacing

It is often suggested that patella tracking after total knee arthroplasty (TKA) with an asymmetrical patella groove is more physiological than with a symmetrical patella groove. Therefore, this study tried to address two questions: what is the effect of TKA on patella tracking, and is patella tracking after asymmetrical TKA more physiological than patella tracking after symmetrical TKA? The patellar and tibial kinematics of five cadaveric knee specimens were measured in the intact situation, after the incision and suturing of a zipper, and after placement of a symmetrical TKA and an asymmetrical TKA, respectively. The patellae were not resurfaced. The flexion-extension kinematics were measured with an internal and external tibial moment to determine the envelope of motion (laxity bandwidth) of the tibio-femoral and patello-femoral articulation. The kinematics after TKA showed statistically significant changes in comparison to the intact situation: patellar medio-lateral translation, patellar tilt and tibial rotation were significantly affected. No statistically significant differences in knee kinematics were found between the symmetrical and the asymmetrical TKAs. We conclude that conventional TKA significantly changes physiological patello-femoral kinematics, and TKA with an asymmetrical patella groove does not improve the non-physiological tracking of the patella.     

Dynamic measurement of patellofemoral kinematics and contact pressure after lateral retinacular release: an in vitro study

The purpose of this study was to investigate the influence of lateral retinacular release and medial and lateral retinacular deficiency on patellofemoral position and retropatellar contact pressure. Human knee specimens (n = 8, mean age = 65 SD 7 years, all male) were tested in a kinematic knee-simulating machine. During simulation of an isokinetic knee extension cycle from 120° to full extension, a hydraulic cylinder applied sufficient force to the quadriceps tendon to produce an extension moment of 31 Nm. The position of the patella was measured using an ultrasound based motion analysis system (CMS 100^®, Zebris). The amount of patellofemoral contact pressure and its pressure distribution was measured using a pressure sensitive film (Tekscan^®, Boston). Patellar position and contact pressure were first investigated in intact knee conditions, after a lateral retinacular release and a release of the medial and lateral retinaculum. After lateral retinacular release the patella continuously moved from a significant medialised position at flexion (P = 0.01) to a lateralised position (P = 0.02) at full knee extension compared to intact conditions, the centre of patellofemoral contact pressure was significantly medialised (0.04) between 120° and 60° knee flexion. Patellofemoral contact pressure did not change significantly. In the deficient knee conditions the patella moved on a significant lateralised track (P = 0.04) through the entire extension cycle with a lateralised centre of patellofemoral pressure (P = 0.04) with a trend (P = 0.08) towards increased patellofemoral pressure. The results suggest that lateral retinacular release did not inevitably stabilise or medialise patellar tracking through the entire knee extension cycle, but could decrease pressure on the lateral patellar facet in knee flexion. Therefore lateral retinacular release should be considered carefully in cases of patellar instability.     

320层动态容积CT对正常人髌骨运动轨迹的初步研究

目的 探讨320层容积CT运动功能成像在正常人髌骨运动轨迹的应用价值.方法 回顾性分析30例应用320层容积CT运动功能成像检查的动态膝关节扫描数据.所有志愿者分别在屈膝0°～120°时行320层CT检测,测量不同屈曲角度时髌骨中点在三维坐标中的位置,分析不同角度时髌股关节排列关系.结果 随膝角的变化,在0°～90°时,髌骨迅速沿Y轴方向(即矢状面)向下移动约(53.87±0.45)mm,随后进入平台期,变化甚微.当屈膝至10°～30°时,髌骨沿X轴向内侧移动达最大程度,范围在(2.31±0.52)～(3.36±0.43)mm,而后向外侧运动,至120°时达到最大值,平均为(8.53±0.44)mm.在Z轴上,初时呈平台期,30°后呈快速下降趋势.在整个屈膝过程中,髌骨的滑行轨迹类似于开口向外的弧形.结论 320层容积CT运动功能成像可以快速、无痛对髌骨运动轨迹进行准确定位.

Abstract：

Objective To evaluate the diagnostic value of tracking movement of normal patellar using volume scan on sensation 320 CT. Method Data of dynamic scans of 30 knees was collected using the motor function of 320 CT and retrospectively analyzed. The data of movement of the patellorfemoral joint was obtained during flexion (from 0° to 120°) within 10-sec by 320 CT from all volunteers. The 3D coordinate of the center of patella was recorded to investigate the dispose relation of patellofemoral joint.Result With the knee angle changed from 0° to 90°, the patella moved rapidly along the Y-axis direction ( sagittal plane) down about (53.87 ± 0. 45 ) mm, and then entered the plateau phase with little change.When the knee flexion reached 10°-30° ,the patellar movement along the X-axis reached the largest range of (2. 31 ±0. 52)-(3.36 ± 0. 43 ) mm, and subsequently moved to the opposite lateral direction with the maximum about (8. 53 ± 0. 44 ) mm at 120°. In the Z axis, the track initially showed plateau, and then presented a rapidly downward trend after 30°. The patellar tracking is like an outward arc during the whole knee flexion. Conclusion The motor functional imaging of 320 CT can pinpoint the patelar tracking in a fast, painless way.     

The influence of medial patellofemoral ligament on patellofemoral joint kinematics and patellar stability

Purpose

Retinacular restraints have a critical role in patellar tracking, limiting the movement of the patella in the trochlear groove. The medial patellofemoral ligament (MPFL) is probably the main stabilizer against lateral displacement; few studies are focused on MPFL role on patellofemoral kinematics and patellar stability. The main goal of this in vitro study was to analyse the influence of the MPFL on the kinematics of the patellofemoral joint and patellar stability.

Methods

Using a non-image-based navigation system, kinematics and anatomical data of six fresh-frozen specimens were collected. A passive flexion–extension from 0° to 90° and static acquisitions at 0°, 30°, 60° and 90°, with and without 25 N of lateral load, were performed with intact and resected MPFL with a 60 N axial force applied to the isolated quadriceps tendon. Patellar tilt and shift were analysed.

Results

The MPFL intact state showed a shift in medial direction during the first degrees of knee flexion—that disappeared in MPFL resected condition—followed by a lateral shift, similar to that of MPFL resected condition. Tilt analysis showed that patella rotated laterally until 85° of knee flexion for intact MPFL condition and until 70° for resected MPFL condition and after rotated medially. Static tests showed that patellar stability was significantly affected by MPFL resected condition in particular at 30° and 60°.

Conclusions

The MPFL has an aponeurotic nature. It works as a restraint during motion, with an active role under high stress on lateral side, but with a small contribution during neutral knee flexion. Its biomechanical behaviour under loading conditions should be kept into account when performing surgical reconstruction of this ligamentous structure.     

Changes to patellar blood flow after minimally invasive total knee arthroplasty

Standard surgical exposure reduces blood flow to the patella during total knee arthroplasty (TKA). Reduction of patellar blood flow has resulted in patellofemoral complications including osteonecrosis and patellar fracture, necessitating revision surgery. In TKA, avoiding patellar eversion is one type of minimally invasive surgery (MIS) technique. This study is the first to measure patellar blood flow during MIS TKA with the knee in both extension and 90° of flexion followed by lateral retraction and then eversion of the patella. Patellar blood flow was measured using laser Doppler flowmetry in 40 patients during MIS TKA. A significant reduction in flow was noted when the leg was flexed from full extension to 90°. Eversion of the patella reduced patellar blood flow. MIS TKA without patellar eversion may be useful for preventing a reduction in patellar blood flow.     

The effects of medialization and anteromedialization of the tibial tubercle on patellofemoral mechanics and kinematics

BACKGROUND: Medialization and anteromedialization of the tibial tubercle are used to correct patellar subluxation in adults. PURPOSE: To compare the effects of the 2 osteotomies on patellofemoral joint contact pressures and kinematics. STUDY DESIGN: Controlled laboratory study. METHODS: Tibial tubercle osteotomies were performed on 10 cadaveric human knees. The knees were tested between 0 degrees and 90 degrees of flexion while dynamic patellofemoral joint contact pressure and kinematic data were simultaneously obtained. Four conditions were tested: normal knee alignment, simulated increased Q angle, postmedialization of the tibial tubercle, and postanteromedialization of the tubercle. RESULTS: An increased Q angle laterally translated the patella, shifted force to the lateral facet, and increased patella contact pressures. Both medialization and anteromedialization partially corrected the abnormal contact pressures. Medialization partially corrected the shift of force to the lateral facet induced by an increased Q angle, whereas the anteromedialization could not. Both medialization and anteromedialization corrected the patella maltracking. CONCLUSION: Medialization and anteromedialization are equivalent in their ability to correct abnormal patellar mechanics and kinematics.     

Reliability and interobserver variability in radiological patellar height ratios

This study evaluated the reliability and interobserver variability of five patellar height ratios as measured by two examiners on standard radiographs: Insall-Salvati (IS), modified Insall-Salvati (MIS), Blackburne-Peel (BP), Caton-Deschamps (CD), and Labelle-Laurin (LL). Plain lateral radiographs with a knee flexion angle of 20° for IS, MIS, BP, and CD ratios and 90° for the LL method of 22 knees of 21 patients with varying pathological knee conditions were analyzed. Statistical results revealed a low interobserver variability with high correlation coefficients (0.86 for IS, 0.82 for MIS, 0.86 for BP, 0.92 for CD, and 0.81 for LL; P > 0.3) and low mean interobserver errors. However, regarding the reliability of the radiographic results of the different methods for patella alta, baja, or norma we found varying results in 68% of the patients. In two patients the patellar height was classified as alta, norma, or baja depending on the ratio used. Regarding the definitions of patellar height used by the authors of these methods, we found the lowest number of normal patellae with the IS ratio and no patella alta for the CD ratio. The LL method revealed the highest number of patella alta. The BP ratio showed intermediate results for both patella alta and baja, being the most moderate method. This study showed that there was a good interobserver reliability for the evaluation of patellar height according to the common radiological ratios. However, the high frequency of differing results between the different radiographic ratios showed that patellar height classification as “alta,”“norma,” or “baja” depends heavily on the chosen index. The differing results were due mainly to the normative patellar height data and to anatomical differences. Based on these findings we recommend a ratio using the articular surface of the patella in relation to the joint line. We recommend the BP method because it revealed the lowest interobserver variability and discriminated best among the groups alta, norma, and baja. Received: 6 December 1999 Accepted: 15 March 2000     

Effects of four different surgical approaches on intra-operative joint gap in posterior-stabilized total knee arthroplasty

Purpose

The effects of surgical approaches and patellar positions on joint gap measurement during total knee arthroplasty (TKA) remain unclear. We hypothesized that joint gap changes with different knee flexion angles would not be consistent within four different approaches and two different patellar positions.

Methods

This study enrolled 80 knees undergoing posterior-stabilized TKA. For 60 varus knees, parapatellar, midvastus, and subvastus approaches were used in 20 knees each. For 20 valgus knees, a lateral subvastus approach was used. Component gap length and inclination were measured intra-operatively using a specific tensor device under 40 lb with the patella reduced or shifted laterally, at 0°, 45°, 90°, and 135° of knee flexion.

Results

Mean gap lengths at 45° and 90° of knee flexion were significantly larger with the parapatellar approach than with midvastus or lateral subvastus approaches (P < 0.05). Regarding gap inclination, varus angle increased linearly through the entire arc of flexion in all four approaches. When the patella was shifted laterally, gap lengths at 45°, 90°, and 135° were significantly reduced compared with those for the patella reduced in the subvastus approach, whereas gap length was constant in the parapatellar approach, regardless of patellar position.

Conclusion

Joint gap kinematics was not consistent within four different approaches and two different patellar positions. Relatively large gaps at 45° and 90° were unique features for the parapatellar approach. Surgeons should be aware that the flexion gap is reduced when the patella is shifted laterally in vastus medialis-preserving approaches such as the subvastus approach.

Level of evidence

II.     

Biomechanical and kinematic influences of a total infrapatellar fat pad resection on the knee

BACKGROUND: This biomechanical study was performed to evaluate the consequences of total infrapatellar fat pad resection on knee kinematics and patellar contact pressure. HYPOTHESIS: Resection of the infrapatellar fat pad produces significant changes in knee kinematics and patellar contact pressure. STUDY DESIGN: Biomechanical cadaveric study. METHODS: Isokinetic knee extension was simulated on 10 human knee cadaveric specimens (6 men, 4 women; mean age at death, 44 years). Joint kinematics were evaluated by an ultrasound-based 3D motion analysis system, and retro-patellar contact pressure was measured using an electronic pressure-sensitive film. All data were taken before and after resection of the infrapatellar fat pad and statistically analyzed. RESULTS: A total resection of the infrapatellar fat pad resulted in a significant decrease of the tibial external rotation relative to the femur between 63 degrees of flexion and full knee extension (maximum: 3 degrees rotation difference at 0 degrees knee flexion, P = .011), combined with a significant medial translation of the patella between 29 degrees and 69 degrees of knee flexion (range, 0.9-1.3 mm, P = .017-.028). Retro-patellar contact pressure was significantly reduced (from 20% to 25%, P = .008-.021) at all flexion angles. CONCLUSION: A resection of the infrapatellar fat influences patellar biomechanics and knee kinematics. CLINICAL RELEVANCE: The infrapatellar fat pad may have a biomechanical function and may play a role in anterior knee pain syndrome.     

Patellofemoral kinematics during deep knee flexion after total knee replacement: a computational simulation

Purpose

Actions requiring deep knee flexion, such as kneeling and squatting, are challenging to perform after total knee replacement (TKR), though many manufactures emphasize that their knee prostheses could safely achieve high flexion. Little is known about the patellofemoral kinematics during deep flexion. This study aimed to track the movement of the patella during kneeling and squatting through dynamic computational simulation.

Methods

A validated knee model was used to analyse the patellar kinematics after TKR, including shifting, tilting and rotation. The data were captured from full extension to 135° of knee flexion. For kneeling, an anterior force of 500 N was applied perpendicularly on the tibial tubercle as the knee flexed from 90° to 135°. For squatting, a ground reaction force was applied through the tibia from full extension to 135° of flexion.

Results

This study found that patellar shifting and rotation in kneeling were similar to those while squatting. However, during kneeling, the patella had a greater medial tilt and showed signs of abrupt patellar tilt owning to an external force being concentrated on the tibial tubercle.

Conclusions

In terms of squatting and kneeling movements, the latter is a more strenuous action for the patellofemoral joint after TKR due to the high forces acting on the tibial tubercle. It is suggested that overweight patients or those requiring high flexion should try to avoid kneeling to reduce the risk of the polyethylene wear. Further modification of trochlear geometry may be required to accommodate abrupt changes in patellar tilting.

Level of evidence

II.     

Plain patellofemoral radiographs

Plain radiographs of the patellofemoral joint can show patellar location, bone shape, trabecular pattern, and articular space thickness. With stress application, stability can be measured. Radiographs may be obtained in the lateral, anterior-posterior, oblique or axial projections, with or without weight-bearing, with or without muscle contraction, with or without stress application to the ligaments, and with the knee in varying degrees of flexion. In addition to trabecular pattern, the lateral radiograph can reveal patellar subluxation, height, or trochlear dysplasia. The axial view is rarely useful until 30° of knee flexion and then may show joint space loss, patellar shift or tilt, and trochlear geometry. Stress radiographs may be required to show how far the patella can be displaced from the trochlea with a given force, and thus, it may be the most accurate method of determining patellar instability. Weight-bearing films may be necessary to show loss of joint space.     

Magnetic resonance imaging of patellofemoral relationships

Patellofemoral relationships were analyzed in 11 patients (13 knees) with patellar dislocation and 15 asymptomatic subjects (15 knees) at 0° and 20° of flexion. The measurements were made from five consecutive axial images through the patellofemoral joint. The six indices measured were lateral patellar tilt (LPT), lateral patellofemoral angle (LPA), lateral patellar displacement (LPD), patella-lateral condyle index (L/PW), congruence angle (CA), and sulcus angle (SA). The reproducibility of the method was evaluated. The difference between the two study groups was more evident at 0° than at 20° of knee flexion. Significant differences were noted between measurements made at different levels of the joint, particularly in the controls. Isometric contraction of the quadriceps muscle lateralized and tilted the patella slightly in both groups. L/PW with and without quadriceps muscle contraction, and LPA with reference to the anterior condyles differentiated between the two study groups most clearly. LPT and LPA with reference to the anterior condyles differentiated the study groups better than LPT and LPA with reference to the posterior condyles. The reproducibility was good except for inter-observer comparison of CA and SA. The use of an imaging plane selected at the midpoint of the patellar articular cartilage increases the sensitivity of the measurements, since it takes into account both the height of the patella and the tendency towards lateralization. These results indicate that patellar tilt is best measured with the LPA index and patellar lateralization with the L/PW index at 0° knee flexion. This study should always include isometric contraction of the quadriceps muscle.     

髌骨轴位X线检查辅助装置的设计与临床应用

目的:设计一套髌骨轴位X线检查辅助装置,以满足膝关节不同程度屈曲受限患者髌骨轴位X线摄影及其临床应用价值.方法:髌骨轴位辅助装置制作材料均使用不锈钢材质,由膝部定位组件和探测器支撑组件两部分组成.本研究纳入100例膝关节髌股关节炎患者,膝关节均有不同程度屈曲受限,使用辅助装置进行髌骨轴位摄影,由2名擅长骨关节影像诊断的高年资影像医师观察髌骨轴位图像,并对其主要解剖结构显示能力及图像质量进行评价.结果:使用髌骨轴位辅助装置首次摄影成功率为98％,优级片率为93％,图像质量符合诊断要求;能够在膝关节屈曲不同角度(15°～90°)下拍摄髌骨轴位,并清晰显示髌骨运行轨迹情况以及准确测量髌骨沟角、适合角等相关参数.结论:使用髌骨轴位X线检查辅助装置简单、易行、摄影成功率高,图像质量符合影像诊断学要求.对于髌股关节炎、髌骨习惯性脱位、髌骨骨折的诊断有一定的临床价值.     

Superior dislocation of the patella with early onset patellofemoral arthritis: a case report and literature review

Among the traumatic patellar dislocations, superior dislocation of patella without patellar ligament injury is very rare. We present in this article, a case of superior dislocation of the patella trapped by interlocked osteophytes in a 38-year-old female who had concurrent bilateral knee arthrosis. Succesful reduction was achieved by closed manipulation without anesthesia. Neither redislocation nor symptoms of instability was seen after 36 months of follow-up, although some progression of arthritis was observed. A new classification including all traumatic patellar dislocation was also proposed.     

Computed tomography of the normal patellofemoral joint

Radiographic assessment of the patellofemoral joint (PFJ) is problematic because conventional views are cumbersome, difficult to standardize, and not reliable for displaying this joint with flexion of the knee less than about 30 degrees. We used computed tomography (CT) to obtain conveniently a direct transaxial view of the PFJ in ten normal human volunteers at different 0 degree, 20 degrees, and 45 degrees of knee flexion and during both contraction and relaxation of the quadriceps muscle. From the CT images we measured femoral trochlear angle, patellar centralization, femoral trochlear depth, and patella tilt angle. In full extension, with the quadriceps muscle relaxed, 19 of 20 knees showed the patella well centered in the femoral trochlear groove. The tilt and centralization of the patella were largely unchanged at 20 and 45 degrees of knee flexion with quadriceps contraction. CT appears to be an optimal method of studying the PFJ. The normal relationships described in this report can be the basis for evaluating patients with known or suspicious history of recurrent subluxation or dislocation of the patella.     

The influence of patella height on intra-operative soft tissue balance in posterior-stabilized total knee arthroplasty

Purpose

Although the patella reduced or everted position has recently been recognized as an important factor in influencing soft tissue balance during the assessment in total knee arthroplasty (TKA), the influence of patella height on soft tissue balance has not been well addressed. We therefore investigated the effect of patella height by comparing soft tissue balance between high [Insall–Salvati index (ISI)?>?1] and low patella patients (ISI?≦?1).

Methods

Using a tensor designed to facilitate soft tissue balance measurements with a reduced patellofemoral joint and femoral component in place, we intra-operatively assessed the joint component gap and ligament balance of posterior-stabilized (PS) TKAs in 30 osteoarthritic patients performed at 0, 10, 45, 90, and 135° of flexion, with the patella reduced.

Results

When comparing the two groups, the component gaps of the higher patella group showed a larger trend than those of the lower patella group, with significant differences at 90 and 135° of knee flexion. Moreover, the joint component gap positively correlated with ISI at 90 and 135° of knee flexion.

Conclusion

Patella higher group showed significant larger component gaps than patella lower group in high flexion angles (90 and 135°). Pre-operative measurement of patellar height can help predict intra-operative soft tissue balance in PS TKA.

Level of evidence

III.     

The relationship of the infrapatellar branches of the saphenous nerve to arthroscopy portals and incisions for anterior cruciate ligament surgery. An anatomic study

The purposes of this study were 1) to carefully define the anatomic distribution of the infrapatellar branches of the saphenous nerve, 2) to provide the surgeon with reliable parameters for where the nerve is most commonly encountered, and 3) to provide specific surgical recommendations to minimize the risk of nerve injury. To accomplish these goals, we dissected 20 cadaveric, fresh-frozen, matched-pair knees. Calipers were used to measure the distance from the nerve to three clinically relevant and easily reproducible landmarks: the inferior pole of the patella, the medial border of the patella at its midpoint, and a point 2 cm medial to the patellar ligament at the level of the joint line. Distances were recorded with the knees in extension and in 90 degrees of flexion to examine the effect of dynamic knee motion on nerve position. We consistently found two main trunks of the nerve that traverse the knee primarily in a medial to lateral but somewhat proximal to distal direction. Because of this, we recommend that incisions for arthroscopy portals be made in a horizontal fashion to decrease the likelihood of nerve injury. Measured from both the inferior pole of the patella and the medial border of the patella, the nerve moved distally with knee flexion. We therefore recommend that incisions across the anterior aspect of the knee be made with the knee in flexion. In 8 of our 20 specimens, the nerve was actually found at the landmark located 2 cm medial to the patellar ligament. This is an extremely high-risk area and should be avoided if possible.