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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° the femur 'rocks' to articulate through the posterior facets. The medial femoral condyle does not move anteroposteriorly with flexion to 110°.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° of this rotation may be obligatory from 0° to 10° flexion; thereafter little rotation occurs to at least 45°. Total rotation at 110° is about 20°, most if not all of which can be suppressed by applying external rotation to the tibia at 90°. The shapes and movements of the femur and tibia as they articulate in the knee have been the subject of anatomical description and debate since 1836. The literature is by now too extensive to review fully. It is cited more completely elsewhere 1 and will be on the JBJS web site (www.jbjs.org. We have examined the non-weight-bearing cadaver knee throughout its range of flexion, extension and longitudinal rotation using MRI. Subsequently, the specimen was dissected in order to confirm our interpretation of the MR images. This investigation offers three advantages compared with dissection alone or radiography. First, the relative positions and shapes of the bones and cartilage in the intact medial and lateral compartments can be established separately but simultaneously at each increment of flexion, a fact crucial to a description of the movement of the knee. Secondly, the menisci and cruciate ligaments can be imaged while the knee is moved with the capsule intact, and thirdly, MRI can be applied without risk to the living knee.In this paper we describe the shapes of the bones in sagittal section and the relative position of the condyles both medially and laterally at certain points in the flexion cycle. It is then possible to deduce the nature of the rotations and translations which occur between the two bones. In principle, this knowledge can be applied to the living knee, as examined by MRI. Materials and MethodsSpecimens were obtained by sectioning the femur and the tibia 15 cm from the joint line without opening the capsule. In total, 24 knees were imaged and dissected to develop the technique described below or to obtain supplemetary information. Six knees were fully imaged for this study. All were from men with a mean age of 43 years (25 to 55). Five were right-sided. None showed abnormalities other than fibrillation of the cartilage.The 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.

In 13 unloaded living knees we confirmed the findings previously obtained in the unloaded cadaver knee during flexion and external rotation/internal rotation using MRI. In seven loaded living knees with the subjects squatting, the relative tibiofemoral movements were similar to those in the unloaded knee except that the medial femoral condyle tended to move about 4 mm forwards with flexion. Four of the seven loaded knees were studied during flexion in external and internal rotation. As predicted, flexion (squatting) with the tibia in external rotation suppressed the internal rotation of the tibia which had been observed during unloaded flexion. The aims of this study were to determine if the living knee could be imaged in a clinically practicable fashion in the same way as the cadaver knee 1 and whether tibiofemoral motion in the living unloaded knee was the same as that in the unloaded cadaver. We then examined the effect of load applied either externally as a tibial torque or by weightbearing and muscle action with and without tibial longitudinal rotation during flexion in the living knee. Subjects and MethodsTwo groups of volunteers were recruited in the Royal Hospital Haslar, Gosport (hospital A: six knees; five male, aged 21 to 32 years) and in St Mary's Hospital, London (hospital B: seven knees; seven male, aged 22 to 35 years). All the knees were normal and the volunteers Caucasian. Table I gives the details.Hospital A. Images during unloaded flexion were acquired using a Picker Outlook 0.23 Tesla Open Access MRI scanner (Picker International Inc, Cleveland, Ohio). A wooden template, which allowed accurate positioning of the knee in various degrees of flexion, was slotted into the scanner couch. The volunteer lay on the template in the lateral position with the knee to be scanned lowermost. The receiver coil was placed around the knee so that it overlaid the femoral condyles. The thigh was held by Velcro straps and a wooden peg was placed behind the popliteal fossa to prevent movement of the femur during flexion.The following images were obtained: four sagittal slices of 10 mm thickness with a 15 mm gap between cuts at -5°( quadriceps contracting), 10°, 20°, 40°, 50°, 90°, 90° (plus flexion against resistance), 90° (plus extension against resistance) and 110°, and 22 slices of 3 mm thickness centred on the intercondylar notch at 0° (quadriceps relaxed), 30°, 60° and 90°. This examination, with a practised radiographer, required about 40 minutes.A template was made by tracing images of the sagittal sections of the lateral and medial tibial condyles from the first scan. This was then overlaid on the monitor to ensure that the same point on the tibia was being scanned each time. Hospital B. MR images were obtained as previously described by Vedi et al. 2 With the volunteer seated, nonweight-bearing images at -5°, 10°, 45° and 90° were obtained with the foot supported by an examiner. At 90°, the knee was then rotated into tibial external rotation (ER) and internal rotation (IR) and an anteroposterior (AP) dra...

MRI studies of the knee were performed at intervals between full extension and 120 degrees of flexion in six cadavers and also non-weight-bearing and weight-bearing in five volunteers. At each interval sagittal images were obtained through both compartments on which the position of the femoral condyle, identified by the centre of its posterior circular surface which is termed the flexion facet centre (FFC), and the point of closest approximation between the femoral and tibial subchondral plates, the contact point (CP), were identified relative to the posterior tibial cortex. The movements of the CP and FFC were essentially the same in the three groups but in all three the medial differed from the lateral compartment and the movement of the FFC differed from that of the CR Medially from 30 degrees to 120 degrees the FFC and CP coincided and did not move anteroposteriorly. From 30 degrees to 0 degrees the anteroposterior position of the FFC remained unchanged but the CP moved forwards by about 15 mm. Laterally, the FFC and the CP moved backwards together by about 15 mm from 20 degrees to 120 degrees. From 20 degrees to full extension both the FFC and CP moved forwards, but the latter moved more than the former. The differences between the movements of the FFC and the CP could be explained by the sagittal shapes of the bones, especially anteriorly. The term 'roll-back' can be applied to solid bodies, e.g. the condyles, but not to areas. The lateral femoral condyle does roll-back with flexion but the medial does not, i.e. the femur rotates externally around a medial centre. By contrast, both the medial and lateral contact points move back, roughly in parallel, from 0 degrees to 120 degrees but they cannot 'roll'. Femoral roll-back with flexion, usually imagined as backward rolling of both condyles, does not occur.

The author's work using magnetic resonance imaging to study the relative movements (the kinematics) of the tibia and femur is reviewed. The description is understood best by reference to comparative anatomy and by dividing the flexion arc into three components. Knee activities take place mainly between 10 degrees and 120 degrees. Over this arc, the articulating surfaces of the femoral condyles are circular in sagittal section and rotate around their center. The medial condyle does not move anteroposteriorly (roll-back does not occur medially). The lateral condyle tends to roll back producing tibial internal rotation with flexion. From full extension to 10 degrees to 30 degrees tibial internal rotation is coupled with flexion. The articulating surfaces medially are a larger radiused anterior femoral facet, which articulates with an upward-sloping tibial facet. Laterally, the femoral condyle rolls forward onto the anterior horn. Flexion beyond 120 degrees only can be achieved passively. Medially, the femur rolls up onto the posterior horn. Laterally, the femur and the posterior horn drop over the posterior tibia.

In 13 unloaded living knees we confirmed the findings previously obtained in the unloaded cadaver knee during flexion and external rotation/internal rotation using MRI. In seven loaded living knees with the subjects squatting, the relative tibiofemoral movements were similar to those in the unloaded knee except that the medial femoral condyle tended to move about 4 mm forwards with flexion. Four of the seven loaded knees were studied during flexion in external and internal rotation. As predicted, flexion (squatting) with the tibia in external rotation suppressed the internal rotation of the tibia which had been observed during unloaded flexion.

The aim of this paper was to study the anatomical relationship between the piriformis muscle and the sciatic nerve with regard to the possibility of neurological deficit after THA. The incidence of anatomical variation of both structures is 15-30% in the literature. The authors studied 91 cadavers and found an atypical relationship in 19 cases (20.9%). In this study individual variations were found with the following frequency: The sciatic nerve exits below the piriformis muscle in 79.1% of the cases. The sciatic nerve separates into two divisions above the piriformis, one branch passing through the muscle, the other below it (14.3%). An unsplit nerve passes through the piriformis muscle in 2.2%. The nerve separates into two divisions above the piriformis, one branch exiting above the muscle and passing along its dorsal aspect, the second exiting distally below the muscle in 4.4%. The most common reasons for sciatic nerve injury in surgery of the hip joint are direct injuries, ischemia of the nerve tissue, compression or excessive distraction of the nerve, compression by bone cement, thermal damage during cement polymerization, injury during THA dislocation, compression by hematoma, bone prominence or an implanted acetabular component. According to the presented anatomical study, overstretching of the nerve itself or its branches in the area of the pelvitrochanteric muscles after their release from their origin can be another mechanism. Such overstretching can appear in the presence of some of the aforementioned anatomical variants.