Schwarz  was honoured for pioneering hip pressure-volume relationship delineation. Our cadaveric model was based on his methodology but with a number of modifications. For example, open anterior approach was utilised as it was more direct, thus decreasing the chance of needle dislodgement. This approach also allowed visualisation of leakage if present. Instillation in 2 ml increment could better delineate the pressure-volume relationship. We rocked the hip after each instillation of fluid to equilibrate the effective hip joint cavity. Without practising such maneuver in our pilot study, the instantaneous intra-capsular pressure was quite high. Increased sample size could narrow the confidence interval. Pressure was measured by a commercial pressure transducer, which had been validated to have high accuracy. 
Regrettably, due to logistic reasons, the hips could not be examined radiologically to rule out intra-articular pathologies which could significantly affect the result. We were also unable to monitor the pressure through the range of motion and in their combination (e.g. 45-degree flexion with full internal rotation and abduction). Moreover, this study suffered from a few drawbacks that limited generalisation of data to in vivo circumstances. Firstly, in a cadaver model, normal soft tissue tension generated by muscle tone could not be restored. Secondly, we could not guard against minute leakage and uneven distribution of fluid inside the hip joint. Thirdly, weight of the leg and the external force applied by the authors in maintaining the defined hip position were not standardised. And since all subjects were elderly Chinese, extrapolating the information to younger age group and other ethnicities should be cautious. Finally, we only elected to test six positions in either coronal or sagittal plane. But the position that yielded the highest and lowest pressure might locate somewhere between the two planes, and with the degree of rotation not tested in our study.
We would like to alert readers on interpreting the sigmoid curve volume-pressure relationship when the hip was positioned in full internal rotation. This finding could be erroneous as the plateau effect was due to our limitation in recording pressure higher than 200 mmHg. Furthermore, leakage might start to occur when the pressure increased to this high level. Having said that, knowing the exact pressure might not be of clinical importance as 200 mmHg is already well above the critical perfusion pressure. [2, 14] Drake et al reported a pressure of 40 mmHg could already jeopardise the femoral head perfusion. 
In neutral hip position, the intra-capsular pressure approached that critical perfusion pressure when 12 ml of 0.9% saline was instilled into the joint (Figure 2). However, in certain hip joint positions, such dangerously high pressure was attained at a much lower intra-capsular volume. Only 6 ml, 8 ml and 10 ml was required to exceed the critical intra-capsular pressure in full internal rotation, full external rotation, and full abduction respectively. The hip capsule, being reinforced by multiple ligamentous condensations, is not elastic under physiological condition. [21–23] Internal rotation of hip particularly tighten the ischiofemoral ligament and lateral arm of the iliofemoral ligament. The iliofemoral ligament of Bigelow is taut on external rotation.  The ischiofemoral ligament checkreins abduction.  Flexion, however, places least tension on these non-yielding ligaments. 
Although the actual volume of effusion or haemarthrosis was never known in clinical setting, our finding suggested a simple measure could avoid undue intra-capsular pressure by paying respect to the hip position. Based on our data on intra-capsular pressure with reference to the hip positions, care should be exercised to avoid skin or skeletal traction of the affected hip in full external or internal rotation. Provided the intra-capsular volume is less than 12 ml, positioning the hip in 45-degree of flexion can confer a safe intra-capsular pressure below 40 mmHg. Clinically, this position can be simply accomplished by resting the affected leg on two pillows or a Thomas splint with Pearson knee attachment. For hip joint with its content estimated to be more than 14 ml, no hip position was found to be able to attain pressure lower than the critical perfusion level. The only way to maintain a safe level of intra-capsule pressure will be continuous aspiration or open drainage.
Although femoral neck fracture was not investigated in our study, we expect the above discussion might not be valid for displaced fracture. Drake et al reported that the volume of blood that could be aspirated from hip with displaced femoral neck fracture never exceeded 5 cc.  Crawfurd also documented that the intra-capsular pressure was higher in Garden  Grade I and II than in Garden Grade III and IV with an average of 66.4 mmHg and 28 mmHg respectively.  Although no concrete explanation was made, it might be in the event of displaced femoral neck fracture, not only the capsule was torn but also the intra-medullary cavity was rendered communicating with the joint proper. The intra-medullary venous system can effectively drain the hemarthrosis. Before further work is done, projecting our finding into this clinical scenario should be cautious.
From our clinical observation, patients with hip disorders were also commonly noted to rest their hips in flexion. No scientific explanation had been made to account for this apart from empirically relating it to preferential spasm of hip flexors. In our study, we demonstrated that in this particular position of hip, the joint conferred the lowest possible pressure. Relaxing the capsular ligaments in hip flexion could be the reason. An in vivo study might provide a better insight in this issue.