Does joint cartilage require energy?

Does joint cartilage require energy? SIR, Geborek et al showed that induced increase of synovial fluid pressure decreases synovial blood flow,' which, as expected,2 decreases synovial fluid oxygen tension.3 In five minutes synovial fluid pressure raised by flexion decreased synovial fluid oxygen tension in 3/10 cases of knee trauma; these three synovial fluids contained few red cells, potential buffers againist hypoxia.4 Even at resting joint angles synovial fluid pressure correlated with synovial fluid hypoxia and, like synovial fluid volume, erythrocytes, and leucocytes also, with synovial fluid lactic acidosis (unpublished). Ischaemia cannot induce such changes unless oxygen and glucose are consumed. We lack data on oxygen and glucose consumption in man, but Bywaters found that each mg of dry weight horse joint cartilage under aerobic conditions consumed 0*01 pI/h oxygen (his Table XIII) and produced 0*171 pI/h of lactate (Table VII), the rates in synovium being 0-8 and 1-7 p1/h respectively.5 In rabbit leucocytes the rates were 4-15 (oxygen) and 22*5 pl/h (lactate).' From the surface areas in one human knee7 the volume of the innermost vascular synovial intima was calculated to be 0-554 cm3 and that of cartilage 44 cm3.8 We calculated that in p.mol/h (a) this cartilage consumed 3-924 oxygen and produced 67-1 lactate (water 80%), (b) the synovial intima 5-93 oxygen and 12-6 lactate (water 70%), and (c) 106 leucocytes (water 75%) 0-0419 oxygen and 0-227 lactate. In each hour (d) 109 erythrocytes consume 0-027 p.mol oxygen and produce 0-384 ,umol lactate.9 In many cases of knee trauma (unpublished) synovial fluid leucocytes and red cells seemed to have consumed more oxygen and glucose than (required by?) the joint cartilage. The regulation of glycolysis in cartilage differs from that in liver as entry of pyruvate into the citric acid cycle is possibly inhibited by acetyl-CoA derived from fat.'0 In normal cartilage most of the energy may be produced by glycolysis, but if the oxygen consumed is used for oxidation of glucose 11'2 (lipids?) about a quarter of the energy may depend on oxygen. In acidotic joints proper energy supply might require more oxygen as glycolysis is depressed by high lactate concentrations. Damage by reperfusion induced oxygen radicals has been stressed and considered.' The synovial fluid oxygen tension dropped, however, at a rate (61 to 41 mmHg in two minutes)3 that might soon have resulted in anoxia and which, at synovial fluid volumes >10 ml, exceeded the calculated consumption by …