Tibiofemoral movement 3: full flexion in the living knee studied by MRI

We studied active flexion from 90 degrees to 133 degrees and passive flexion to 162 degrees using MRI in 20 unloaded knees in Japanese subjects. Flexion over this arc is accompanied by backward movement of the medial femoral condyle of 4.0 mm and by backward movement laterally of 15 mm, i.e., by internal rotation of the tibia. At 162 degrees the lateral femoral condyle lies posterior to the tibia.     

Tibiofemoral movement 1: the shapes and relative movements of the femur and tibia in the unloaded cadaver knee

In six unloaded cadaver knees we used MRI to determine the shapes of the articular surfaces and their relative movements. These were confirmed by dissection. Medially, the femoral condyle in sagittal section is composed of the arcs of two circles and that of the tibia of two angled flats. The anterior facets articulate in extension. At about 20 degrees the femur 'rocks' to articulate through the posterior facets. The medial femoral condyle does not move anteroposteriorly with flexion to 110 degrees. Laterally, the femoral condyle is composed entirely, or almost entirely, of a single circular facet similar in radius and arc to the posterior medial facet. The tibia is roughly flat. The femur tends to roll backwards with flexion. The combination during flexion of no anteroposterior movement medially (i.e., sliding) and backward rolling (combined with sliding) laterally equates to internal rotation of the tibia around a medial axis with flexion. About 5 degrees of this rotation may be obligatory from 0 degrees to 10 degrees flexion; thereafter little rotation occurs to at least 45 degrees. Total rotation at 110 degrees is about 20 degrees, most if not all of which can be suppressed by applying external rotation to the tibia at 90 degrees.     

Tibiofemoral movement 4: changes of axial tibial rotation caused by forced rotation at the weight-bearing knee studied by RSA

We studied the knees of 11 volunteers using RSA during a step-up exercise requiring extension while weight-bearing from 50 degrees to 0 degrees. The findings on weight-bearing flexion with and without external rotation of the tibia based on MRI were confirmed.     

Tibiofemoral alignment and the results of knee replacement

We examine the hypothesis that a knee replacement is most likely to survive successfully if it is stable with a coronal tibiofemoral angle close to 7 degrees of valgus, the accepted normal. The records of 428 knee replacements followed up for one to nine years were analysed. The highest success rate was indeed found in those so aligned at operation and such knees were most likely to remain stable. Nevertheless, half of the failures occurred in knees correctly aligned at operation and two-fifths in knees which had remained stable in this alignment; many failures must have been caused by factors other than malalignment. Some knees, well aligned at operation, deteriorated into severely varus or valgus positions; their failure rate was significantly higher than that for knees which remained normally aligned and higher also than for knees severely varus or valgus from operation onwards. Malalignment, in itself, may not be the most important cause of failure, though it probably does compound failure from other causes.     

Tibiofemoral conformity and kinematics of rotating-bearing knee prostheses

Increasing tibiofemoral articular conformity theoretically increases articular contact area and reduces contact stresses in total knee arthroplasty. Fixed-bearing knee designs possess relatively low tibiofemoral conformity, in part to allow tibiofemoral rotation without generating excessive stresses at the articulation or the implant-bone interface. This study analyzed knee kinematics of mobile-bearing designs in a closed chain dynamic knee extension model in posterior cruciate-retaining design with high- and low tibiofemoral conformity and posterior cruciate-substituting designs with and without rotational constraint. Overall, for all conditions, the mobile-bearing insert rotated with the femur in the presence of tibiofemoral axial rotation. In addition, the correlation of bearing rotation with femoral rotation was stronger for the high-conformity and rotationally-constrained designs than for the low-conformity designs and strongest for the posterior cruciate-retaining high-conformity condition. Changes in conformity or rotational constraint did not appear to affect femoral roll back, tibiofemoral axial rotation, or varus-valgus angulation. The results suggest that mobile-bearing inserts rotate with the femur and increasing conformity or rotational constraint in mobile-bearing design knee prostheses does not affect knee kinematics adversely, at least under closed chain knee extension conditions in vitro.     

膝关节屈曲时腘动脉移位的MRI研究

[目的]阐明膝关节屈曲时胫骨后方腘动脉(PA)位置的变化。[方法]应用MR检查,分别在关节0°伸直位和90°屈曲位对16个膝进行扫描,在胫骨的两个水平,相当于全膝关节置换术(TKR)和胫骨高位截骨术(HTO)的截骨水平,测量腘动脉到胫骨后方皮质的距离(骨—动脉距离,BAD),用配对t检验比较距离的变化。[结果]从伸直位到屈曲90°位,在TKR水平,BAD平均增加1.8 mm(-2~+5 mm),改变有统计学意义(P=0.005);在HTO水平,BAD平均增加1.4 mm(-2~+4 mm),改变也有统计学意义(P=0.005)。但在两个水平,各有2例表现为腘动脉的前移。[结论]屈曲膝关节是一个相对安全的体位,但在膝部手术中,并不能保证规避损伤腘动脉的潜在危险。     

Response of chondrocyte subpopulations cultured within unloaded and loaded agarose

Although it is well known that the metabolism of chondrocytes can be altered by the application of mechanical strain, it is unclear whether chondrocytes from the superficial and deep zones of cartilage respond in a similar manner. In this study, chondrocytes from the uppermost 15-20% (superficial cells) and the remaining tissue (deep cells) from bovine articular cartilage were isolated separately and cultured in agarose constructs. Cell deformation on application of a 15% static compressive strain was identical for both subpopulations after 24 and 72 hours in culture. The constructs were incubated under static and dynamic (0.3,1, and 3 Hz) strains of 15% amplitude. Glycosaminoglycan synthesis by deep cells was unaffected by static strain or 3 Hz dynamic strain, whereas 0.3 Hz produced a significant reduction and 1 Hz induced a highly significant 50% stimulation of glycosaminoglycan synthesis (p < 0.001). Superficial cells exhibited a general inhibition of glycosaminoglycan synthesis. By contrast, proliferation of superficial cells was stimulated by dynamic strain whereas deep cells were not influenced. It has been suggested previously that mechanotransduction-induced controls of glycosaminoglycan synthesis and proliferation in chondrocytes embedded in agarose are uncoupled. Data presented in this study demonstrate that the two processes do, in fact, occur in different sub-populations of chondrocytes within the full-depth cell isolate.     

Measurement of human lumbar spine ligaments during loaded and unloaded motion

Length changes in the ligaments of human lumbar spine motion segments were investigated in order to find conditions under which unusual stress or stress reduction is found in the longitudinal ligaments and facet joint capsules. Flexibility measurements were performed under load. Increasing load and height reduction in the motion segment increases the flexibility. The anterior and posterior longitudinal ligament normally operate in the elastic part of their stress-strain curve. Destruction of the intervertebral disc leads to a dislocation of the centers of rotation in the motion segment. Two types of facet joint capsules were observed. Height reduction in the motion segment leads to abnormal strains in one type sooner than in the other, which can be normalized by slight flexion. Injection of a silicone compound into the disc normalizes many of the changes in the motion segment due to height reduction.     

Tibiofemoral contact stress after total knee arthroplasty: comparison of fixed and mobile-bearing inlay designs

We measured tibiofemoral contact stresses and the load-bearing contact area of fixed and mobile-bearing inlay knee prostheses under dynamic loading conditions. An electronic resistive pressure-measuring sensor was used to detect contact stresses and contact area in five cadaver knees. Stresses were measured with the tibial component aligned normally, as well as in internally- and externally-rotated positions. The average peak contact stresses measured on the fixed inlay were greater (medial 21 MPa and lateral 21 MPa) than those on the mobile inlay (medial/lateral 7.7/5.3 MPa, p = 0.04). Although the average peak contact stresses of the fixed standard inlay greatly exceeded the contact stresses of the other two inlay designs in each malrotated position tested, no statistically significant differences were seen. The data suggest that the ability of the inlay to translate on the tibial baseplate permits the inlay to align itself on the femoral component so that the contact surface area is maximized and contact stresses are reduced.     

Tibiofemoral contact stress after total knee arthroplasty: Comparison of fixed and mobile-bearing inlay designs

We measured tibiofemoral contact stresses and the load-bearing contact area of fixed and mobile-bearing inlay knee prostheses under dynamic loading conditions. An electronic resistive pressuremeasuring sensor was used to detect contact stresses and contact area in five cadaver knees. Stresses were measured with the tibial component aligned normally, as well as in internally- and externally-rotated positions. The average peak contact stresses measured on the fixed inlay were greater (medial 21 MPa and lateral 21 MPa) than those on the mobile inlay (medial/lateral 7.7/5.3 MPa, p = 0.04). Although the average peak contact stresses of the fixed standard inlay greatly exceeded the contact stresses of the other two inlay designs in each malrotated position tested, no statistically significant differences were seen. The data suggest that the ability of the inlay to translate on the tibial baseplate permits the inlay to align itself on the femoral component so that the contact surface area is maximized and contact stresses are reduced.     

Erratum: Tibiofemoral force following total knee arthroplasty: Comparison of four prosthesis designs in vitro

The authors have inadvertently left out several acknowledgments and wish to add them here This study was supported by DePuy International, the University of Western Australia, the Fremantle Hospital, and Mrs. Huggler. We would like to thank Dr. Hilaire Jacob for his help with the development of the mini‐force platform. We would also like to thank the Australian Orthopaedic Association (AOA) for the financial support of Dr. Jeffcote.     

Tibiofemoral force following total knee arthroplasty: comparison of four prosthesis designs in vitro.

Despite ongoing evolution in total knee arthroplasty (TKA) prosthesis design, restricted flexion continues to be common postoperatively. Compressive tibiofemoral force during flexion is generated through the interaction between soft tissues and prosthesis geometry. In this study, we compared the compressive tibiofemoral force in vitro of four commonly used prostheses: fixed-bearing PCL (posterior cruciate ligament)-retaining (PFC), mobile-bearing posterior-stabilized (PS), posterior-stabilized with a High Flex femoral component (HF), and mobile-bearing PCL-sacrificing (LCS). Fourteen fresh-frozen cadaver knee joints were tested in a passive motion rig, and tibiofemoral force measured using a modified tibial baseplate instrumented with six load cells. The implants without posterior stabilization displayed an exponential increase in force after 90 degrees of flexion, while PS implants maintained low force throughout the range of motion. The fixed-bearing PFC prosthesis displayed the highest peak force (214 +/- 68 N at 150 degrees flexion). Sacrifice of the PCL decreased the peak force to a level comparable with the LCS implant. The use of a PCL-substituting post and cam system reduced the peak force up to 78%, irrespective of whether it was a high-flex or a standard PS knee. However, other factors such as preoperative range of motion, knee joint kinematics, soft tissue impingement, and implantation technique play a role in postoperative knee function. The present study suggests that a posterior-stabilized TKA design might be advantageous in reducing soft tissue tension in deep flexion. Further research is necessary to fully understand all factors affecting knee flexion after TKA.     

Posterior malleolar ankle fractures: an in vitro biomechanical analysis of stability in the loaded and unloaded states.

The use of internal fixation for posterior malleolar fractures remains controversial. This study assessed the contribution of the posterior malleolus/posterior tibiofibular (PM/PTF) complex to ankle stability in the loaded condition. Total plantarflexion and dorsiflexion, torsion, version, and drawer were measured with 15- and 70-kg loads before and after creation of posterior malleolar fractures that measured 25% of the distal articular surface. After internal fixation, specimens were retested. Fractures caused significant increases with internal rotation and posterior drawer at 15- and 70-kg loads in the neutral position. External rotation, anterior drawer, and dorsiflexion increased at 15-kg load. Fixation increased stability of fractured specimens, but not significantly. Fractures of the PM/PTF ligament complex may lead to excessive internal rotation and posterior instability in the loaded state.     

慢性压迫性颈脊髓病的手术疗效与MRI表现分析

目的：分析慢性压迫性颈脊髓病的MRI表现,探讨术前预测慢性压迫性颈脊髓病手术疗效的意义。方法：对115例慢性压迫性颈脊髓病患者的MRI和术后半年以上复查的81例MRI图像,选椎管矢径最窄节段脊髓进行分辨、测量和对照,并依据脊髓功能40分评分法对疗效进行评定。结果：髓内有边界清楚局限性T1WI低或等信号、T2WI高信号灶的26例,术后玉术前MRI各测量值皆无显著性差异（P＞0.05）,术后改善率＜30％。髓内无上述异常信号灶的89例,其术后MRI各值及改善率与术前髓断面积、髓断面积与管断面积比值大小呈正相关。结论：术前髓内有无边界清楚局限性T1WI、T2WI异常信号灶、髓断面积的大小、髓断面积与管断面积比值可作为预测手术疗效的依据。