FIELD OF THE INVENTION The present invention relates to a knee orthosis pre- venting hyperextension, i. e. excessive extension of the knee joint. The orthosis features a two-axes joint allowing a natural joint motion and a popliteal connection bar provid- ing the required stability. The orthosis is light-weight and made of few components. The orthosis allows easy donning and doffing by incorporating adequate straps.

STATE OF THE ART The first version of the hyperextension orthosis con- sisted of a rigid brace without any hinge or joint. Thus, it did not bend with the leg which, of course, was impractical.

This orthosis was replaced by an orthosis having a single axis joint. Even though this was an improvement over the rigid brace, the single axis joint did not follow the natu- ral motion of the knee joint. The orthosis was provided with a stabilizing connection bar in the popliteal space. When the patient bent the knee joint, the leg pinched around the connection bar. Thus, the movement of the patient's leg was limited and experienced as uncomfortable. Furthermore, the orthosis was secured to the leg by ventral and dorsal straps attached to the orthosis at the same height. Due to this ar- rangement, the orthosis was subject to slipping down the leg.

The present invention solves the problems stated above by providing a knee orthosis comprising a two-axes joint promoting a natural motion. Also, the ventral and dorsal straps are arranged at different heights and preferably off- set from a centre line. This arrangement distributes the forces and torques on the orthosis in an advantageous manner preventing slipping.

SUMMARY OF THE INVENTION Thus, the present invention provides a knee orthosis comprising a top portion to be secured at the patient's thigh and a lower portion to be secured at the patient's lower leg, a joint portion connecting the top and lower por- tions, and strap means for securing the orthosis. According to the invention the joint portion has two axes preventing hyperextension but otherwise allowing a free motion of the patient's leg.

Preferably, the joint portion comprises two hinges con- nected together by a connection bar positioned in or near the popliteal space.

In a preferred embodiment, the top and lower portions comprise brace members having ventral and dorsal slits for the strap means. The ventral and dorsal slits are positioned at different heights and may be offset from the centre line of the orthosis.

The invention is set forth in greater detail in the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be explained further in detail with reference to the accompanying drawings in which: Figure 1 is a side view of a knee joint orthosis ac- cording to the present invention secured the leg of a pa- tient, Figure 2 is a detail view of a brace member of the or- thosis of Figure 1, Figure 3 is a side view of a joint plate and a hinge end of a connection bar, and Figure 4 is a top view of a portion of a connection bar of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS As is mentioned in the introduction, the present inven- tion relates to a hyperextension knee joint orthosis, i. e. an orthosis preventing excessive extension of the knee joint. Indications are a generally weak knee joint with in- stable knee, especially in case of hyperextension caused by laxity of the ligaments, and genu valgum or genu varum.

With reference to Figure 1, the knee orthosis generally consists of a top portion and a lower portion connected by a joint portion. The top portion comprises brace members 1 and two ventral straps 3 and 4 and two dorsal straps 7 and 8.

Similarly, the lower portion comprises brace members 2 and two ventral straps 5 and 6 as well as two dorsal straps 8 and 9. Conveniently, as is shown, the lower dorsal strap of the top portion and the top dorsal strap of the lower por- tion consists of the same strap 8 which is passed through slits 19 and in a cross at the back of the knee, i. e. in the popliteal space. The joint portion comprises a joint plate 10 and a connection bar 14.

Only one side of the orthosis and the leg is shown but it will be understood that the orthosis comprises the iden- tical corresponding parts at the other side. Besides, one and the same orthosis may easily be adapted for the right or the left leg of a patient.

One brace member is shown in detail in Figure 2. All four brace members are preferably identical in shape. As may be seen, slits 19 are provided for the various straps. Also the brace member has a pivot 20 to provide a pivot axis of the joint together with the joint plate 10.

In Figure 3 the joint plate 10 is shown to the right, while the hinge end 15 of the connection bar 14 is shown to the left. The joint plate 10 provides a top axis 11 and a lower axis 12 together with the respective brace members 1 and 2. The axes are spaced by approximately 50 mm. The dis- tance is not very critical. Inside the joint plate, the end portion 15 of the connection bar is attached by rivets 13 or the like. The end portion 15 constitutes a stop portion gov- erning the motion of the brace members 1 and 2. The stop portion has a top stop surface 17 and a lower stop surface 18 which engage corresponding stop surfaces 21 (Figure 2) on the brace members. Except for the stop thus provided, the top and lower brace members 1,2 can move freely and inde- pendently of each other.

The orthosis allows a full extension of approximately 170° and a bending of the leg to a full contraction at ap- proximately 15°. It will be appreciated by persons skilled in the art that full extension angles greater or less than

170° easily may be obtained by adapting the stop surfaces 17,18,21. Also, at full contraction, often the muscles of the patient's legs prevents further contraction. The sole desired function of the orthosis is to prevent hyperexten- sion and the other extreme of the leg bending movement is not of particular interest but the orthosis should only fol- low the natural movement of the knee joint as close as pos- sible, such that the patient does not experience any discom- fort or resistance except at the extension stop.

For a correct operation of the joint portion the joint plates 10 are connected by a connection bar 14 maintaining the two joint plates (of which only one is shown in Figure 1) in parallel. The connection bar consists of two hinge portions 15, as is described above, and a popliteal stabi- lizing bar 16. The popliteal bar consists of a flat portion that is angled relative to the vertical. This is to increase the stabilizing effect and strength of the bar. It is also more comfortable when the patient bends his leg around the connection bar 14 in that no edges of the connection bar are pressed into the leg.

It is advantageous that the connection bar 14 has a variable width so that the orthosis fits different widths of legs. An adjustable variable width of approximately 10- 14.5 cm accommodates the majority of the grown up popula- tion. In Figure 4 the connection bar 14 comprises two parts sliding along each other but other arrangements are evident to a person skilled in the art. The connection bar 14 is se- cured by connecting screws. The most adequate way for this connecting bar is to use two screws to secure the width and stability of the medial and lateral joint.

As is mentioned previously, the orthosis has three dor- sal straps 7,8,9. These straps do not have to be removed for donning and doffing the orthosis. The dorsal straps will be adjustable for circumference by means of pile and loop connections (e. g. VELCRO tapes). On the other hand, the ventral straps 3,4,5,6 are provided with pile and loop connections or the like to be easily opened and removed and put back on again. In the illustrated embodiment of the orthosis, four ventral straps are provided but, in fact, only three straps are needed and the ventral strap 4 above

the knee is not strictly needed for securing the orthosis but gives the patient a comfortable feel.

The slits 19 for the ventral and dorsal straps are ad- vantageously arranged at different heights, the top ventral strap 3 and lower ventral strap 6 being at the highest and lowest positions, respectively. The slits 19 are also offset from a vertical centre line of the orthosis. This arrange- ment results in that the forces applied by the straps and acting at the slits 19 produce torques acting on the brace members. These torques assist in holding the orthosis to the leg in a way which were not possible if the slits 19 were arranged at the same height. In a relaxed position the dis- tal part of strap 8 and strap 5 produce a ring around the smallest circumference of the leg, namely over the Gastroc- nemius. Thus, the orthosis is prevented from slipping down the patient's leg and the weight of the orthosis rests mainly on the large calf muscle (M. Gastrocnemius).

As is indicated above, all the brace members are made from identical plates having the same punched holes. Thus, the number of components to be manufactured is kept as low as possible. The brace members are preferably produced from a 2 mm aluminium sheet providing the required strength and low weight. The brace members may be bent'manually, or with a tool, to adapt the orthosis to the patient's leg, the top brace members 1 usually being bent outwards to accommodate the relatively thicker thigh and the low members 2 being bent inwards, or contoured after the dimension of the leg, if necessary.

All the hinges of the orthosis may be produced from a 2 mm aluminium sheet. However, it is preferred that the con- nection bars 14 and the joint plates 10 are made of stain- less steel because of its greater strength.

Thus, the present hyperextension knee orthosis presents several advantages over the prior art orthoses. The two-axes joint portion allows for a free joint motion of 170° to 15° while following the natural movement of the patient's knee joint as close as possible. The incorporated straps make handling easier. The orthosis has a slim design and is light-weight (approximately 300 g). The orthosis does not migrate (slip) because of the adequate positioning of the

straps. The popliteal connection bar keeps the essential parts in parallel.

While the invention has been set forth in great detail, it is appreciated by persons skilled in the art that the or- thosis may be subject to various modifications and rear- rangements without departing from the scope of the invention which is defined in the accompanying claims.